A big problem for me is I can't just open to a random page and start reading to see if it's worthwhile. I have to jump over to Google Books, and that can be iffy as to whether or not they have samples I can read. Google Books also isn't good about searching by title, and I often have to use the author's name in combination or search by ISBN, and I don't always have those. Google Books does have the useful facet of searching by release, so if a book's been re-released under a new publisher or with a new foreword or something it'll tell me that.