What is the Apocrypha and Pseudepigrapha?
Pseudepigrapha:
spurious or pseudonymous writings, especially Jewish writings ascribed to various biblical patriarchs and prophets but composed within approximately 200 years of the birth of Jesus Christ. They are works attributed to a given author but could not have been writen by them and thus are somewhat dubious in authentication/authority.

Apocrypha:
The word apocrypha means "hidden" in Greek. In Judaism and early Christianity, this term was originally applied to important teachings that were to be kept hidden
from the general public. However, by the fourth or fifth century A.D., the Christian world of that time had largely rejected the idea of special or "hidden" teachings and thus rejected any books that claimed to preserve such teachings.
  
Consequently, during this period the word apocrypha came to have a new meaning--"heretical" or "rejected." Over the centuries the terms apocrypha or apocryphal have again been amended through usage, and are now understood to mean "not scriptural." In this broad sense any religious writing that claims divine inspiration but which is not found in the scriptures is called "apocryphal."

The major importance of the Pseudepigrapha is that they come from around the time of Jesus and therefore provide a background of popular culture and religious beliefs against which to place our study of the Old and New Testaments. The Apocrypha and Pseudepigrapha together partly fill in the historical, cultural, and religious gap that otherwise exists in our knowledge of the period between the testaments. They also reveal that some of the beliefs and practices of Judaism and Christianity at the time of Christ were quite different from what they became shortly thereafter. 

D&C 91:1-6

1 there are many things therein that are true - and they are mostly translated corectly
2 but there are many things that are false and are interpolations of men
3 it is not needful that the apocrypha should be translated
4-5 whoso readeth it let him understand - and be enlightened by the spirit and obtain benefit from them
6 whoso receiveth not the spirit will gain no benefit from them

Rule Books

Manual of Discipline - lays down rules of personal conduct
War Rules - instructs the community how to conduct the final battle
Temple Scroll -Scroll discusses rules of ritual cleanness and uncleanness

Wisdom Literature

Biblical Commentaries

Fenesis Apocryphon -a retelling of the stories of Noah and Abraham and commentaries on the books of Habakkuk, Isaiah,
Nahum, Micah, and others. Most of these are fragmentary
A short work on Melchizedek
Copper Scroll

In late 1945, a collection of thirteen papyrus books was dicovered in a jar buried near the town of Nag Hammadi in Upper Egypt. These thirteen books, or codices,
contained a total of forty-four separate works written in the Coptic language. Most of these date from between the late second and early fith centuries A.D. and are the product of a kind of Christianity called Gnosticism.

The Second Treatise of the Great Seth
Apocryphon of John
On the Origin of the World
Testimony of Truth
the First Apocalypse of James
Book of Thomas the Contender