I suppose Webfiction or Webnovels are not technically Books. But it is a large and growing market with a lot of excellent reads in it.

Many of them more popular or more succesfull than "real" published works you can get actual physical copies of.

I understand not wanting to shove all of Royalroad, Watpadd and AO3 into your books database or becoming an indirect fanfiction review site but needing an ISBN or Goodreads entry to exist seems too limiting.