When I was fading from JWs one of the books I read was The Crucifixion of Truth by Tony Bushby.
I probably don't have to read any more than this. Tony Bushby plain makes stuff up; he is one of the most unreliable sources I have ever seen pertaining to biblical matters.
This document is the apocalyptic writing of the Sibyl of Tarquin, originally called The Mysteries of Osiris and Isis. It was the true origin of the Book of Revelation and the probable source of the concept of the Essene's two most precious prayers to the Heavenly Father and Earthly Mother.
I did some checking and what Bushby calls "The Mysteries of Osiris and Isis" is simply his name for what Edmond Bordeaux Szekely called the "Essene Book of Revelation" and that book (text online here) originated with Szekely; it is a modern-day apocryphon. Szekely produced quite a few similar works in his time that he claimed were ancient but which lacked any manuscript or ancient evidence whatsoever (see my comments about his "Essene Gospel of Peace"). I think it is quite clear looking at the text that the source of the "Essene Book of Revelation" is Revelation itself. And the identification of this text with the lost books of the Cumaean Sibyl (but which partially survive in Virgil, Ovid, and Petronius) is simply an assertion by Bushby.
One interested in these matters must be careful to rely on genuine ancient sources and not modern writings; there is a plethora of recent fakes available online.
Now there are real genuine extrabiblical writings that could be postulated as influences, if not sources, of Revelation. 1 Enoch is one pretty obvious work that exerted its influence on Revelation, particularly in its concepts of a subterranean abyss imprisoning angels with chains and the eternal punishment of the resurrected wicked with fire. Another work that is a probable source for Revelation is the Zoroastrian apocalypse called the Oracles of Hystaspes which, if the excerpts preserved by Lactantius are reliable, shows some very striking similarities with ch. 11 and 13 of Revelation. This book was widely read by Christians, as Justin Martyr attests, and it was banned by the Roman government under penalty of death (i.e. it was anti-Roman propaganda much like Revelation). The chiliasm of the book, found also widely in Phyrgia and elsewhere in Asia Minor (attested also in Papias, Cerinthus, Irenaeus via Polycarp, and the later Montanists), is probably of Zoroastrian origin as well. The mythological scene in ch. 12 is likely rooted in the Isis-Horus/Leto-Apollo myth, which separately is utilized in the 13 Kingdoms portion of the enigmatic Apocalypse of Adam. This work is thought by some to be proto-gnostic and quite early (antedating second-century Sethian gnosticism), and it also has influences from Mithraic and other mythologies; I think it is thus possibly a work utilizing similar traditions as ch. 12 of Revelation, much like some of the Sibylline Oracles draw on material closely related to ch. 13-17. I should also mention 4 Ezra, a non-Christian Jewish apocalypse written around the same time as Revelation, and which has striking parallels with material in both the synoptic apocalypse (Mark 13) and Revelation. The strongest influence however is the OT; Revelation is the most allusive book of the OT in the whole NT. Almost every verse repackages some motif or phraseology or concept drawn freely from the OT but especially from the Prophets.
