The Necronomicon is a book with demonic powers featured in Jason Goes to Hell as well as Freddy vs. Jason vs. Ash. It was believed to be able to revive Freddy, who was trapped inside Jason's sub-conscious.
Given its location in the Vorhees home, it is likely that this is also how Jason's mother brought him back to life after he drowned in a lake as a child, starting the whole series of events followed by the movies.
The Necronomicon was originally designer by H.P. Lovecraft and books by the name have appeared in many works since. The book appearing in Jason Goes to Hell used the same design as seen in the Evil Dead series, hinting at a possible cross-over between Evil Dead and Friday 13th.