General

I would also like to know of any books

I decided to do a quick search on Amazon before posting and found a book called “the ultimate weight gain guide” by Barbara T. Davis, no idea how informative this one would be

This seems interesting enough... pity it's not likely available in a brick-and-mortar store for anonymity.

“How to Get Fat”
Probably not exactly what the OP is looking for, as it’s more of a tongue-in-cheek novelty aimed at women than a legitimate guide—but it’s not entirely devoid of useful information and suggestions. I bought it a long time ago and left it permanently on my nightstand as a sort of subliminal encouragement and motivator. It didn’t hurt.