No, the Necronomicon is a work of fiction dreamed up by H.P. Lovecraft, and put into print by some nut(s) who evidently thought it would be cool.
Nietzsche (typed it wrong the 1st time) was an existentialist philosopher and author -- also a nutcase, by the way -- who famous works include Thus Spake Zarathustra, Human, All Too Human, and Beyond Good and Evil.
Existentialism is hard to define briefly, but the gist of it is this:
The is no inherent purpose to life; thus, our lives are meaningless except for the meaning that we ourselves give them. Or something like that. It's the "one thing" that Curly talked about in City Slickers, "That's what you have to find out!"
Even if you are devoutly religious, you can be an existentialist if that religion is your raison d'etre (reason for being). The point is to live and have a purpose, because without a purpose, you got no reason to live.
No wonder some people find it depressing...
--the Chaos Turtle
The writer's only responsibility is to his art. He will be completely ruthless if he is a good one. ... If a writer has to rob his mother, he will not hesitate; the "Ode on a Grecian Urn" is worth any number of old ladies.
--William Faulkner, American author and honorary Pro Tour playa