Thanks to a post by John D. Reid on his Canada's Anglo-Celtic Connections blog, I learned about the books printed between 1473 and 1700 now online curtesy of the University of Michigan libraries. These are not digitized books, but the full text transcriptions of the contents. Here is a description of the newly added text files from the University of Michigan Library:
The texts of the first printed editions of Shakespeare, Chaucer, and Milton as well as lesser-known titles from the early modern era can now be freely read by anyone with an Internet connection. The University of Michigan Library, the University of Oxford’s Bodleian Libraries and ProQuest have made public more than 25,000 manually transcribed texts from the first 200 years of the printed book (1473-1700). These texts represent a significant portion of the estimated total output of English-language work published during the first two centuries of printing in England.
The release (via Creative Commons Public Domain Dedication) marks the completion of the first phase in the Early English Books Online-Text Creation Partnership (EEBO-TCP). An anticipated 40,000 additional texts are planned for release into the public domain by the end of the decade.
Full-text public access to the transcribed EEBO-TCP texts is hosted by the U-M Library at quod.lib.umich.edu/e/eebogroup. The Bodleian offers individual text downloads in several formats, including ePUB files.The contents of the new offerings on both the Oxford University and ProQuest offerings are only available to registered users. Fortunately, the list of the books is available without registration on the Oxford University website and the list and the contents can be searched on the University of Michigan website.
If you have been hoping to push your genealogical efforts back to the 1600s and even the 1500s, you can get a good idea of what would be entailed in such an effort from looking at a few of these books. You can also search online for digitized copies of these books or other text transcriptions.