At some point of the Graveyard Keeper, you will get a quest from Snake; he will want to acquire Necronomicon. There's only one way to obtain it - The Lighthouse Keeper will give it to you in exchange for moths.
To get the moths you have to go to a meadow at night and simply collect the flowers. Moths will be added to your equipment along with them. If you do this during the day you will catch a few butterflies that are useless in this case.
Then, you just need to go to the Lighthouse Keeper and give the moths to him in exchange for the book for Snake.