There's no doubt that anyone who has been doing any online genealogy research has at one time or another done a search on Google Books. If not, this is a rich resource that no family history researcher can ignore.
First, access the site at http://books.google.com/ From here, it works just like searching Google. Once you locate a resource you want to "bookmark" - you just add the book to your online library. You can also identify bookshelves by topic so you can locate your books faster.
Some of the books that are indexed on the site have no preview or limited preview due to copyright restrictions. However, there are links to assist users in locating the book for sale or in a library.
Here are just a few of the gems that have assisted my research:
Ancestry magazine - back issues are available to read online
Men of Minnesota - I found biographies and photos of a related Kelly family here
History of Ramsey County and St Paul, Minnesota - since several of my Kelly family lived in St Paul for a while, this was a great view of the city and Ramsey county
History of Waterbury, Connecticut - my Welch family lived here, so I wanted to learn more about this community.
A Lost Lady by Willa Cather - Who would think that a novel would aid in family history research? Well, the notes to this scholarly edition of the book has a reference to my great-great grandfather, although not by name. He is referred to as the father-in-law of John Fitzgerald and it tells me that he hauled freight from Omaha to Denver. This is certainly a resource that I doubt I would have found if not for Google books.
Illustrated History of the State of Nebraska - By far, my best "hit" so far - this gives a fairly detailed biography the great-great grandfather referenced in A Lost Lady - William D. Kelly. This is the book where I first learned the name of his father and that the family came from Kilkenny, Ireland.
This is just a sampling of the dozens of books I've located with family history information. If you haven't done so already, search for your own family members on Google books.