I use it to research the histories of areas that aren’t well-represented online, and to check dates when I don’t quite trust Wikipedia. Some books show up in the results as full page scans with searchable text. Other books are restricted to just showing a few preview pages or a few paragraphs of excerpts. Some are downloadable as PDF documents. (Even if you can’t see all of the information, Google gives you the publisher’s information that gives you a head start on finding it at your library.)
Because I have a fairly uncommon surname, Dobush, I tried searching for it. Google Book Search turned up some academic works by people with my last name, as well as some Jewish history books (which is intriguing, because that side of my family is Catholic as far as I know). But the best find was a 1916 book titled “Songs of Ukrania: With Ruthenian Poems.”
The book’s old enough to be in the public domain, and I was able to download a PDF of it. There in the index, under the subheading Robber Songs, is an epic poem titled “The Death of Dobush.” It describes an Alexa Dobush as a Carpathian Robin Hood who stole from the rich to give to the poor. Leads to chase for that side of my family tree just got a lot more interesting!