Google Books is an online service from Google that searches the full text of books that Google has scanned, converted to text using optical character recognition, and stored in its digital database.
Value for Family History Researchers
The value of the Google Books service for family history and genealogy researchers can be substantial. An instructor at BYU's Salt Lake Center gave an assignment to find something in a particular book at the Family History Library. One student decided to look up a particular book as part of the assignment, which was given in the materials for the assignment. The book was found to be missing at the Family History Library, so the student turned to Google Books. That book was found there, along with 19 other books by the same author.
Types of books that are often found by family history researchers include town histories, other histories about places, both in general and specific, some actual family histories that were printed and that are now out of copyright, and even historical records held by some archives, such as some in Europe that are actively working with Google to digitize materials and make them available via Google Books.
For more detailed information about the Google Books site and service, see the Wikipedia article which is found by way of the graphic on this page from Wikipedia.