noun
treasured book; secret book; pornographic book
Refers to a book kept hidden or treasured, often with esoteric, rare, or erotic content. The pornographic sense is a common modern extension.
祖父の書斎から古い秘本が見つかった。
An old treasured book was found in my grandfather's study.
彼は秘本を集めるのが趣味だ。
His hobby is collecting secret or rare books.