Ben Witherington has posted the following notice on his blog.
"The Princeton Theological Seminary Library has recently made available
online full texts of some 80,000 volumes, so of which, one would
presume, are now out-of-print works, but still valuable for certain
research purposes. Still there are plenty of resources here of value.
Key point— you need to have the critical judgment to know which is a
valuable resource, and which is merely an old book."
Two points of clarification. (1) The exact number of books is 76,717, 26,783 of which come from the Princeton library. (2) The link on Witherington's blog is incorrect as is pointed out in the comments section (the link below should be correct). In any case, the search engine allows you to refine your search by date, format, and language.
You can access the page and books here.