FAQ

Because he's an idiot. A lovable idiot. But an idiot nonetheless. Bruce Campbell and Sam Raimi often joke about how capable Ash can be. But also dumb Ash can be.

In terms of the character's motivations in the show. We see Ash smoke a joint and show off the book to a girl to impress her. In his altered state of mind, he starts reading passages. It's also possible Ash wanted the evil to return, because his life of being an overweight, middle-aged retail worker who lives in a trailer isn't exactly how he saw his life turning out. Ash felt capable and confident when fighting the deadites. Which gave him a purpose.

The show follows Evil Dead 1 and 2. Evil Dead 2 is referenced the most. Army Of Darkness is never referenced due to rights issues. It could be said that the events still happened, but Ash never bothers mentioning his travelling through time because nobody ever believed him. This is confirmed when he nonchalantly mentions getting sucked in to a rift that sent him back to medieval times. Much to the bewilderment of the other characters.

In the original Evil Dead, Ash throws the book of the dead into the fireplace, it appears to burn and destroy the remaining two deadites. However if you watch the scene closely, you see while the book reacts to fire, we never see it actually being destroyed. The book's eyes open and a tongue extends from it and that's the last we see of it. In Evil Dead 2, the Necronomicon is seen sitting on a table, Ash briefly reads from it and then it isn't seen again (although the first 7 minutes of Evil Dead 2 are supposed to be recap and not exactly canon) for the rest of the film, they use the lost pages of the book. So in terms of continuity, it could be said that Ash threw the book in the fire and assumed it was destroyed. Eventually someone else came across the cabin, found the book and sold it. Ash somehow learned of this and tracked to book down and acquired it for safe keeping.