Oh gosh, what kind of book are we talking about here, like, fiction, non-fiction, reference? Also I have a very hard time picking out what "favorite" even means...

On the fiction side, I would say I am very much enjoying a trashy web-novel called 「かませ役♂に憑依転生した俺はＴＳを諦めない」currently.

On the non-fiction side, "High-Speed Digital Design: A Handbook of Black Magic" is pretty good, but I don't know how obscure it is.