You're reading something that only a real maniac would read.
Only someone who's really into that would read this book.
ーYou're reading something that only a real maniac would read.
「マニアしか読まないような本読んでるね。」
something that only a real maniac would read で「マニアしか読まないようなもの（本）」
ーOnly someone who's really into that would read this book.
「この本はそれにハマってる人しか読まないよ。」
only someone who's really into ... で「…にハマっている人だけ」
ご参考まで！
カナダ 
