The thought occurred to me to try and determine how many digitized books are now (as of February, 2010) online. Then, I would try to estimate/guess how many of those are genealogy related.
I realize that the task is likely impossible, but I would like to have some idea of the magnitude of the online book community. My first stop is Google Books. It appears that Google doesn't publicize the actual number of books scanned. But an article in the Official Google Blog of October 9, 2009 gave the total at more than 10,000,000. Doing a quick search in Google Books on the subject of "genealogy" returns about 26,000 books, while a search on "family history" returns about 17,900. Google Books has three levels of online availability; Limited Preview, Snippet View and Full View. Only Full View gives the entire book's contents.
It is obvious that many more books than just those appearing in a search for "genealogy" have content valuable to family historians and researchers, but it is apparent that the Google project is not at all directed at the subject area of family history and any books digitized will be incidental to Google's overall project.
Let's jump to the Family History Archive. As of February, 2010, the combined libraries of the Family History Archive had digitized and published online 57,125 books and manuscripts. All of these books are exactly on point pertinent to someone's family history.
On to Archive.org. This huge digital resource claims 150 billion pages scanned. However, a search on the term "genealogy" returns about 12,184 items. Archive.org does have a self-described "ever expanding collection of genealogy resources including items from the Allen County Public Library..." Since Archive.org includes all types of media, it is difficult to determine how many of the items are really related to genealogy and family history. This is a resource worth looking at since it appears to contain a lot of local histories, yearbooks, directories and other pertinent publications.
Here are a few other sites:
The Online Book Page -- claims over 35,000 free books from the University of Pennsylvania.
Gutenberg -- claims over 30,000 free books
Digital Book Index -- claims over 148,000 full-text digital books.
OK, this doesn't have much to do with genealogical research but the best site I found was the International Children's Digital Library. I really loved the poetry books in Spanish.
As I searched, I quickly began to understand that there was no visible end to the mountains, no, whole worlds of books online. This turned out to be a really difficult project because I kept finding interesting things to look at.
So when I got all the way through, I began to realize, I will never get to the end of this. There are more books going online than I have time to search.