I would look first at the FamilySearch.org Books link to the Brigham Young University Family History Archive. The link is on the main startup page for FamilySearch.org. The number of books in the digitized collection is a complete mystery since the counter has been non-functional since last year some time. In addition to surname books, the collection also has:
- Histories of families
- County and local histories
- How-to books on genealogy
- Genealogy magazines
- Periodicals (including some international)
- Medieval books (including histories and pedigrees)
The second place to search is Google.com Books. Try putting in the surnames and the word genealogy or family or history. A search for Tanner genealogy returned 10,200 results, including many public domain books that were available to read or download. It might take you a while to exhaust this vast collection. If the books are not online in a readable format, you can add them to your Google Books account and purchase a digital copy.
Next try Archive.org. I didn't find very many under a search for Tanner genealogy, but I found quite a few with a search for Tanner. If you strike out with Archive.org, try searching in WorldCat.org and look for online editions of the books you find.
When you get through with those searches, you might just try a Google.com search for related topics. I searched for "tanner genealogy books online digitized" and got some interesting results. You might even run across my recent post on reading genealogy books online - a survey of sources.
Now, you might worry that some of these "sources" might, in fact, be drivel. That is entirely possible. But, I would suggest that you at least become familiar with what some of your relatives may have done before. Many of the older books are ascendancy books and the information about the progenitors may be suspect but a lot of the information may have been gathered first hand by the author. Once in a while, you hit a gold mine of information tied to contemporary sources. Good luck in your searching.