I'd appreciate some clarification, just in case I'm missing key game mechanics.
Some spells require mastery of a section of the Necronomicon. Does mastery come by reading once? If not, what would count as mastery? Must the book still be used to cast mastered spells? I could not find any Pathfinder rules about mastering spellbooks.
What if a spell, like contact entity, is not on a class's list? Does the Necronomicon impart special ability to still cast it?
Thank you.
I'm just another player/DM but here are my opinions.
If you are an arcane caster with a spellbook, after one successful reading you can transcribe them, ie you have mastered it. If you're a spontaneous arcane caster you can automatically learn these spells the next time you have a free slot to learn or substitute a spell.
If the spell isn't on your spell list, or you aren't an arcane caster you are out of luck, reading the Necronomicon doesn't override other rules. If I was a DM I would apply the above to psychic casters. Mine didn't, and that's OK too.