I believe that's the reference name they go by in many places, as a way to avoid: 'Pornographic' as a word. Either that, or because most books like them are typically very thin, perhaps not even gathering 6 sheets of paper.
Well, doujinshi are often just 20~30 pages long, including character introduction, artist's pre/after words, guess illustration, go to 38~40 pages. And those are mostly nonH doujin. THe erotic ones focus on the act of having sex after all. So, some are even 8~14 pages with just sex. The illustration ones don't have above 20 pages either.
Of course though, thin books are what they use to call the erotic doujins.
Those are compilations. And I hardly know any tankoubon that is actually under a title as doujin. Except for titles started as independent doujin than gain name on their own.
I know!
Alrighty, let's do a book that I haven't let anyone else see yet, shall we!Commander...
W-W-W-What is this...Oh, if I must... I'll read a book to you so you'll go to sleep.Tokitsukaze... You're getting up early tomorrow, you know.It's an adult picture book~
THIN
BOOK
Commander~
I can't sleeeep~Buuuut~Yaaaay!!Hey now, no roughhousing.Hurray~!
What kind!? What kind!?Really~!? wham
wham