First, nothing is really "missing" from the Bible. The books that were later selected to go into the canon came from a broader literary context, but it was a selection process that determined what ended up going into the Bible. So many of the apocryphal gospels, for instance, that were written in the second century were not even contenders for inclusion in the second-century proto-orthodox canon, so they are hardly "missing" from it (tho some of these were indeed "missing" from world literature until they were rediscovered). What I mean is that there are many different canons -- at least a dozen of them -- a dozen different anthologies with different subdivisions that represent different results of the selection process in different religious communities (e.g. the Roman Catholic canon, the Greek Orthodox canon, the Nestorian Church canon, the Ethiopian Orthodox Church canon, the Armenian canon, the Georgian canon, the Russian Orthodox canon, the Anglican canon, the Protestant canon, etc. etc.). These represent different decisions made at different times on what counts as canonical. Of course, this does mean that some books were intentionally excluded, even banned.
Because the books in the current Protestant canon were plucked from an original wider literary context, they may utilize scriptures that themselves were not accorded canonical status in the history of this canon; the use of 1 Enoch in Jude is just one notable example (see my thread on this: http://www.jehovahs-witness.com/10/85223/1.ashx), although this book is still canonical in the Ethiopian Orthodox Church (along with Jubilees and 4 Ezra, the latter book being included in Anglican Bibles in the Apocrypha under the name "2 Esdras", cf. the King James Version, the Revised Standard Version, and the New English Bible).
If you are interested in reading some of this broader context, I could recommend the following books: 1) The Context of Scripture (which has Egyptian, Babylonian, Sumerian, Canaanite, Israelite texts that form part of the OT's background), 2) N. Wyatt's Religious Texts From Ugarit (which has all Canaanite religious texts from Ras Shamra), 3) The 1966 unabridged Jerusalem Bible (which has all the Apocrypha), 4) the two volumes of Charlesworth's Old Testament Pseudepigrapha, 4) either Martinez' The Dead Sea Scrolls Translated: The Qumran Texts in English (which includes the Hebrew/Aramaic text) or Wise, Abegg, & Cook's The Dead Sea Scrolls: A New Translation, 5) Miller & Funk's The Complete Gospels, 6) Holmes' The Apostolic Fathers: Greek Texts and English Translations, and 6) Layton's The Gnostic Scriptures. Most of these are available on Amazon, and most can be found at good university libraries.
Finally, there are sources mentioned in the OT (like the Book of Jashar) which are no longer extant -- assuming that they existed in the first place -- but one must be careful not to confuse them with much later midrashic books written under their names. The pseudepigraphal Sefer ha-Yashar, for instance, is still valuable as a source of rabbinic tradition, not as a book that formed a source to the Bible, and it must not be confused with the nine other works circulating under the same name.
