Why Am I Sweaty?

 is a book that, from the front cover's blurb, "Your body explained in horrifyingly uncomfortable detail."

History
Grunkle Stan catches Mabel (in Dipper's body) and mistakenly believes he's "at that creepy age where you spy on girls." So he takes her to his office and forcibly has Mabel sit and listen to him while he explains puberty and the birds and the bees. The book starts off with the pituitary glad and goes on to explain where babies come from.

Excerpt
"Puberty is the greatest mystery of all Also: go outside and make friends"