In Revelation, immediately after John eats the “little book”, he is told by the angel who carried it that he must “prophesy again before many peoples, and nations, and tongues, and kings”.
The use of that word “again”, and the context of this being something that John must do in the future leads me to think that if the “little book” is any book of the New Testament, it would Revelation itself. “Again” implies he had prophesied already to many peoples in the past, which could refer to writing prophecies in a previous gospel, or simple prophesying as part of his evangelical mission prior to his exile.
Also, Revelation is the only book in the NT that is composed almost entirely of prophecy like the writings of the OT prophets. After John had this vision, in which he was given a book and told he must prophesy again, he wrote THIS book of prophecy...
OK, well it appears we will be able to ask him about this shortly. MARANATHA! ;-)