What exaxctly are we getting (in terms of content and quantity) with a Google scanned book?

I’m curious to know how the Google algorithm searches for Google book content and quantity. I think I remember reading that a result offers the reader 10% of an entire book but in my experience the content will often have gaps in the pages. Is the Google search algorithms only selecting content within the entire book absolutely relevant to the imputted query ? If I search the same book using other criteria, will I get different content from that same book?