Meaning of “Usui hon(薄い本)”

“Usui hon(薄い本)” means fanzine or erotic fanzine as a slang.
“Usui(薄い)” means thin and “hon(本)” means book.
So the literal meaning of it is just a thin book.
Most of fanzin is thin, that’s why the meaning of it is fanzin as a slang.

Example Conversation

A: Yesterday, my mother tidy up my house…
B: Yeah
A: I think she found my Usui hon(薄い本) collection…
B: Kusa(草)! You got caught to being Fujoshi(婦女子)!
※Fujoshi(腐女子) means a girl who likes boys love.

A: 昨日親が私の部屋掃除してさぁ
B: うん
A: 私の薄い本コレクションが大量に見つかった恐れがある…
B: 草！腐女子バレしたね！