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Kalled2Preach stated,

If the gifts had ceased, or were even on their way out, why would this book have been accepted as true and part of the canon? Why weren't other prophecies which may have been written before Hebrews or before Revelation and didn't contradict what they already accepted as Scripture included?
I thought this was kind of obvious. We are discussing “new” revelation, however if it agrees with what they already accepted as Scripture—it would not be “new,” would it? Second, and more importantly, for something to have been accepted as canon in the early church it would have had to been attested to by apostolic authority.

The Book of Hebrews was in much contention for its acceptance into the canon and was among the Antilegomena (7 disputed books) and not the Homologoumena (20 accepted books). The earliest list of NT books of which we have definite knowledge was drawn up by the heretic Marcion about 140. It consisted of an (a) expurgated edition of Luke, and (b) 10 of the Pauline Epistles. The Church of course protested and it moved it to over a period of time to develop a correct canon (Luke 12:12; John 16:13). Metzger, states,

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The Montanist heresy caused the Church to develop a mistrust of all recent writings of a prophetical nature. Not only did such a feeling tend to discredit several apocalypses that may have been, in various parts of the Church, on their way to establishing themselves, but even the Revelation of John was sometimes brought under a cloud of suspicion because of its usefulness in supporting the 'New Prophecy.'

Writing in about 96 AD, Clement emphasized the importance of apostolic authority, stating, "The apostles received the gospel for us from the Lord Jesus Christ; Jesus the Christ was sent forth from God. So then Christ is from God, and the apostles from Christ. Both, therefore, came of the will of God in good order." Thus, later the Church used this as its test. Thus, if a valid “new” revelation, which did not violate what was considered true Scripture, but was not attested to by an Apostle, it was considered suspect and not included in the canon. The Church became extremely cautious and since the Apostles can no longer attest to any revelation—revelation is closed! There can be no new revelation.

Warfield draws upon much more Church history and surmises that the NTcanon was imposed by the apostles on the Church. Thus the canon was not only complete but also fully and finally accepted by the end of the first century. In an article originally published in 1892 Warfield writes, "In every case the principle on which a book was accepted, or doubts about it laid aside, was the historical tradition of apostolicity." However, "the principle of canonicity was not apostolic authorship, but imposition by the apostles as 'law.'" Warfield then further explains, “The authority of the apostles, as by divine appointment founders of the Church, was embodied in whatever books they imposed on the Church as law, not merely in those they themselves had written.” However, it is important to note that the Church did not originate the Bible. Its inspiration is divine, not ecclesiastical. It stands or falls because of its relationship to God, not to the Church. The Church never attempted to confer canonicity. The Church did not give authority to the canon, rather it simply recognized its authority.

For more details on this I would suggest reading, The Canon of the New Testament: Its Origin, Development, and Significance by Metzger. Another book you may enjoy is THE NEW TESTAMENT DOCUMENTS. The Highway has several good articles as well;

The Formation of the Canon of the New Testament

Brief History of Divine Revelation

What is the Word of God?

Reformed and Always Reforming,