Watson's Biblical & Theological Dictionary 
which occur under the same Hebrew word with books, namely, ספר , are mentioned the more rarely, the farther we go back into antiquity. An epistle is first mentioned, 2 Samuel 11:14 , &c. Afterward, there is more frequent mention of them; and sometimes an epistle is meant, when literally a messenger is spoken of, as in Ezra 4:15-17 . In the east, letters are commonly sent unsealed. In case, however, they are sent to persons of distinction, they are placed in a valuable purse, which is tied, closed over with wax or clay, and then stamped with a signet,
Isaiah 29:11; Job 38:14 . The most ancient epistles begin and end without either salutation or farewell; but under the Persian monarchy the salutation was very prolix. It is given in an abridged form in Ezra 4:7-10; Ezra 5:7 . The Apostles, in their epistles, used the salutation customary among the Greeks; but they omitted the usual farewell at the close, namely, χαιρειν , and adopted a benediction more conformable to the spirit of the Christian religion. St. Paul, when he dictated his letters, wrote the benediction at the close with his own hand, 2 Thessalonians 3:17 . He was more accustomed to dictate his letters than to write them himself.
The name Epistles is given, by way of eminence, to the letters written by the Apostles, or first preachers of Christianity, to particular churches or persons, on particular occasions or subjects. Of these the Apostle Paul wrote fourteen. St. James wrote one general epistle; St. Peter two; St. John three; and St. Jude one.
An epistle has its Hebrew name from its being rolled or folded together. The modern Arabs roll up their letters, and then flatten them to the breadth of an inch, and paste up the end of them, instead of sealing them. The Persians make up their letters in a roll about six inches long, and a bit of paper is fastened round it with gum, and sealed with an impression of ink, which resembles our printers' ink, but is not so thick. Letters, as stated above, were generally sent to persons of distinction in a bag or purse; but to inferiors, or those who were held in contempt, they were sent open, that is, unenclosed. Lady M. W. Montagu says, the bassa of Belgrade's answer to the English ambassador going to Constantinople was brought to him in a purse of scarlet satin. But, in the case of Nehemiah, an insult was designed to be offered to him by Sanballat, in refusing him the mark of respect usually paid to persons of his station, and treating him contemptuously, by sending the letter open, that is, without the customary appendages when presented to persons of respectability. "Futty Sihng," says Mr. Forbes, "sent a chopdar to me at Dhuboy, with a letter of invitation to the wedding, then celebrating at Brodera at a great expense, and of long continuance. The letter, as usual, from oriental princes, was written on silver paper, flowered with gold, with an additional sprinkling of saffron, enclosed under a cover of gold brocade. The letter was accompanied with a bag of crimson and gold keem-caub, filled with sweetscented seeds, as a mark of favour and good omen."
Easton's Bible Dictionary 
It is an interesting and instructive fact that a large portion of the New Testament is taken up with epistles. The doctrines of Christianity are thus not set forth in any formal treatise, but mainly in a collection of letters. "Christianity was the first great missionary religion. It was the first to break the bonds of race and aim at embracing all mankind. But this necessarily involved a change in the mode in which it was presented. The prophet of the Old Testament, if he had anything to communicate, either appeared in person or sent messengers to speak for him by word of mouth. The narrow limits of Palestine made direct personal communication easy. But the case was different when the Christian Church came to consist of a number of scattered parts, stretching from Mesopotamia in the east to Rome or even Spain in the far west. It was only natural that the apostle by whom the greater number of these communities had been founded should seek to communicate with them by letter."
Bibliography Information Easton, Matthew George. Entry for 'Epistles'. Easton's Bible Dictionary. https://www.studylight.org/dictionaries/eng/ebd/e/epistles.html. 1897.
Smith's Bible Dictionary 
Epistles. ( Letters; Personal Correspondence By Writing). The twenty-one Epistles of the New Testament took the place of tracts among us. In their outward form, they are such as might be expected from men who were brought into contact with Greek and Roman customs, themselves belonging to a different race, and so reproducing the imported style with only partial accuracy.
They begin, (the Epistle to the Hebrews and 1 John excepted), with the names of the writer and of those to whom the Epistle is addressed. Then follows the formula of salutation. Then the letter itself commences in the first person, the singular and plural being used indiscriminately. When the substance of the letter has been completed, come the individual messages. The conclusion, in this case, was probably modified by the fact that the letters were dictated to an amanuensis [A person whose employment is to write what another dictates.]
When he had done his work, the apostle took up the pen or reed, and added in his own large characters, Galatians 6:11, the authenticating autograph. In one instance, Romans 16:22, the amanuensis in his own name adds his salutation.
An allusion in 2 Corinthians 3:1, brings before us another class of letters which must have been in frequent use in the early ages of the Christian Church, by which travellers or teachers were commended by one church to the good offices of others.
Morrish Bible Dictionary 
The name given to the twenty-one 'Letters' (for this is the signification of the word επιστολή, and which is often thus translated) of the New Testament. Each epistle should be regarded as a letter, and be read as a whole. The word is twice used in a figurative sense. Paul said that the saints at Corinth were his 'epistle' written in his heart. They were living examples of Paul's doctrine which could be known and read of all men. The genuine power of his work was being exhibited in them. They were also manifestly the 'epistle of Christ.' By means of Paul, the Spirit of the living God had written Christ upon the fleshy tables of their heart, just as surely as God's finger had written the law on tables of stone. 2 Corinthians 3:2,3 .
Kitto's Popular Cyclopedia of Biblial Literature 
In directing our inquiry first of all towards the relation in which the Epistles stand to the other component parts of the New Testament, we find that both the Old and New Testament have been arranged by divine wisdom after one and the same plan. All the revelations of God to mankind rest upon history. Therefore in the Old, as well as in the New Testament, the history of the deeds of God stands first, as being the basis of Holy Writ; thereupon follow the books which exhibit the doctrines and internal life of the men of God—in the Old Testament the Psalms, the writings of Solomon, etc. and in the New Testament the Epistles of the Apostles; finally, there follow in the Old Testament the writings of the prophets, whose vision extends into the times of the New Testament; and at the conclusion of the New Testament stands its only prophetic book, the Revelation of John.
In this also we must thankfully adore divine wisdom, that the Epistles, which lay down the doctrines of the Christian religion, originate, not from one Apostle alone, but from all the four principal Apostles; so that one and the same divine truth is presented to our eyes in various forms as it were in various mirrors, by which its richness and manifold character are the better displayed.
The Epistles of the New Testament divide themselves into two parts—the Pauline and the so-called Catholic.
The Pauline Epistles are thirteen in number; or fourteen, if we add to them the Epistle to the Hebrews. The very peculiar character of the Pauline Epistles is so striking as to leave not the least doubt of their genuineness. Depth of thought, fire of speech, firmness of character—these manly features, joined withal to the indulgence of feelings of the most devoted love and affection, characterize these Epistles. The amiable personal character of the Apostle may be most beautifully traced in his Epistles to the Philippians and to Philemon.
All the Epistles, except the one to the Romans, were called forth by circumstances and particular occasions in the affairs of the communities to which they were addressed. Not all, however, were preserved; it is, at least, evident, from , that a letter to the Corinthians has been lost; from , it has also been concluded—though probably erroneously, since there perhaps the letter to the Ephesians is referred to—that another letter to the community of Laodicea has likewise been lost. Press of business usually compelled Paul—what was, besides, not uncommon in those times—to use his companions as amanuenses. He mentions , as something peculiar, that he had written this letter with his own hand. Paul himself exhorted the communities mutually to impart to each other his letters to them, and read them aloud in their assemblies . It is therefore probable that copies of these letters had been early made by the several communities, and deposited in the form of collections.
The letters of Paul may be chronologically arranged into those written before his Roman imprisonment, and those written during and after it; thus beginning with his first letter to the Thessalonians, and concluding with his second to Timothy, embracing an interval of about ten years (A.D. 54-64). In our Bibles, however, the letters are arranged according to the preeminent parts and stations of the communities to whom they were addressed, and conclude with the Epistle to the two bishops and a private letter to Philemon.
The Catholic Epistles.—There is, in the first instance, a diversity of opinion respecting their name: some refer it to their writers (letters from all the other Apostles who had entered the stage of authorship along with Paul); some again, to their contents (letters of no special but general Christian tenor); others, again, to the receivers (letters addressed to no community in particular). This last opinion is most decidedly justified by passages from the ancient writers. The Pauline Epistles had all their particular directions, while the letters of Peter, James, I John, and Jude were circular epistles. The Epistles II and III John were subsequently added, and included on account of their shortness, and to this collection was given the name Catholic Letters, in contradistinction to the Pauline.