When Google Books creates images of books, they include every single page including the cover and the blank pages. If the book has been signed by the author or by a donor, that will be included in the scan. Some books are printed with a signature of at least one person involved with or connected to the book. These signatures are typically accompanied by an explanation of their relationship to the other.
A perfect example is Bishop Chase's Reminiscences: An Autobiography. The copy that Google Books scanned was given to Harvard University by George Dexter, a former tutor there. Another book, A Genealogy of the Leavenworth Family in the United States, With Historical Introduction, Etc., was printed with signatures of Thomas Leavenworth's widow, brother and son. The Life of Benjamin Franklin: Containing the Autobiography bears the note, "The Gordon Lester Ford Collection, Presented by his sons, Worthington Chauncer Ford and Paul Leicester Ford to the New York Public Library." Each of the three Ford's appear to have penned their own signature.
Often books featuring a photograph of the subject will include a signature that is probably authentic. An example is The Rawson family. Memoir of Edward Rawson, Secretary of the Colony of Massachusetts Bay, from 1651 to 1686; with Genealogical Notices of His Descendants. It includes photographs and signatures of Reuben Rawson Dodge and Edward Rawson.
Unfortunately, there is no simple way to find these books among the thousands on Google Books. You can, however, narrow down the results by using the Google Books advanced search. Search on the exact phrase "genealogy of" to generate a list of books with those words in the title.
Beyond that, you'll need to open each file. If there is a signature, either printed or handwritten, it will usually be on one of the very first pages in the book. Keep in mind, some books start with the Roman numeral "i" instead of "Page 1." Then you will need to browse through the pages to locate the signature. The search tool for the book does not include the signatures.
One reason this is a good source is that when someone donates a volume to a library, a note will be made about who the donor was and if the donation was made in honor of someone. Libraries keep these notes in the books, even if they are on plates glued to the pages.
So, if you are quite patient and don't mind browsing through a lot of books, you can find a treasure of handwriting samples on Google Books.