Why does Google Scholar results categorize some books as [BOOK] and some as [CITATION] - what's the difference? In addition, why are there apparently no citations to some well-known books even though Google includes these in books.google.com? I.e. they should show up in Google Scholar search results, but they don't.
I can only muse about the inner workings and motivations of Google, but:
- Google Books is for all kind of books, not only scholarly books. And since deciding whether a book is scholarly is something that cannot be automatised just like that, it is arguably less bad to miss some scholarly books which are in Google Books than to have all of Google Books’ books in Google Scholar.
- As already indicated this answer (hinted at by mhwombat), being categorised as citation in Google Scholar essentially means that the only trace Google Scholar has of this work (in a scholarly context) is that it is cited by some other work. In particular, Google does not even know that this piece of work is a book and not any other type of publication (e.g., if you export such a citation to BibTeX, its type defaults to article).
- Google could link its databases and include all books in Google Books in Google Scholar if it manages to match them with a citation, but this might not be such a good idea either as non-scholarly publications are also cited sometimes – just look up some random, very popular pieces of fiction and see whether they got cited.