I mention this book as only one of many examples of works some people claim equal the Bible. I am sure you have come across other examples; the Bible plus the writings of so and so. Is the Bible enough for us? Do we need another source of truth, or is the Bible sufficient to thoroughly equip us for faith and life?
It goes back to my two questions in an earlier posting: is the Bible inspired and is it authoritative? If it is, then it must also be sufficient, unless God has given us some other writings to supplement it. Islam teaches that the Bible was an early revelation, but that the Koran is both additional and superior revelation. Cult leaders usually make the same claim concerning their literary works. One large branch of the church actually includes additional sections, which they call the Apocrypha, in their version of the Bible.