usui hon 薄い本

In Japanese, usui hon 薄い本, meaning literally "thin book," is an anime slang that refers to a doujinshi 同人誌, often, but not necessarily, a doujinshi featuring adult content, i.e. hentai 変態.

Meaning

The usui hon comes from the thickness of a doujinshi book, which is often 30 pages or fewer, really thin compared to the typical manga 漫画 volume that's over a hundred pages long.

Normally, the term revers to an adult doujinshi.^{(paradisearmy.com)}

The reason for the doujinshi to be so thin is that these are books made by hobbyists, one-chapter stories sold individually.

Normally, a manga is serialized in a magazine together with various other stories from various other artists, so you'll have one chapter of manga, one chapter from another, etc., compiled together into a single, large, thick volume.

After serialization, multiple chapters of a manga may be compiled into a single volume, called a tankoubon 単行本, in order for the story to be sold alone.

Compared to this typical sort of manga volume, a doujinshi can be very thin, so it makes sense that doujinshi sometimes get called "thin books."