Some of these websites will be commercial and have a subscription charge, but even those with a charge for some books may also have a selection of free books. In some cases, you may have to dig down into the website to find the digital books, but they will be there. I finally decided to stick with English language websites. I might try some of the other language websites some time, but the difficulty is determining if they are legitimate or pirate sites without knowing the languages that well.
In making this list, it is apparent that there are several large websites that provide online ebook services to libraries. For example, the website Overdrive.com provides a centralized service of digital books only to libraries. Individual users of the Overdrive system gain access to the books in their own public library's online collection through logging in with their library card number and a password. Is this an online digital book collection? That is a good question. The number of books available to the subscribers is substantially different from institution to institution. The question is whether Overdrive is the online website or the local public library? Depending on how you view this question and many others, the number of online websites is determinant or indeterminate.
Another example is Amazon.com. It has millions of digital books, mostly for sale. It also has a "free library" of books. Is it a library or a bookstore? Or both?
It became immediately evident that the availability of ebooks is changing rapidly. Many of the websites listed have some sort of restriction on access. They are either university libraries where only those associated with the school can access the books or they are private companies or repositories with other restrictions. My goal was to make a list long enough and inclusive enough that you can find a way to acquire access to the material you are searching for. Unfortunately, the list became so tangled that I soon realized that this particular goal was unobtainable.
No list of digital books would be complete without a reference to Google Books. Google has clearly amassed the largest online collection of digital books in the world. Some time ago, Google estimated that the total number of books, counting every book ever published to be, 129,864,880. Of course that was back in 2010 and millions more have likely been published since then. The real question is how many of those books has Google digitized? The last estimate was made in April of 2013 and claimed that Google had digitized more than 30 million books. We can assume that Google is still out there digitizing books, so there are probably millions more on the website at any time after 2013. Trove.nla.gov.au, the website of the National Library of Australia, has a published number of books of 18,920,532 as of the date of this post. A search in Trove's book collection for the term "genealogy" results in 137,242 hits. The same search on Google Books results in 7,720,000 results. In either case, the number of books is unimaginably large.
For genealogists, the online collection of digital books on FamilySearch.org includes the content of several large genealogical libraries. The total number of books is well over 100,000 and growing. When you go to the FamilySearch.org website, click on the Research tab at the top of the page and then click on books.
One of challenges of this project is to figure out how to list the websites. I began with the idea to start by listing all the ones I use and/or know about and then move on to all that I can find online anywhere in the world. I have explained what happened at this point. The numbers of books or items listed for some of the websites is more of an indication of their size rather than a current number of holdings. Most of these online libraries continue to acquire more books and other items and the numbers will change daily.
Here is a representative list of online book websites, not in any particular order:
- Internet Archive -- 6,454,689 digitized books, all in the public domain and freely downloadable including over 144,000 books from the Library of Congress
- Open Library -- borrow, read and browse over 2,700,000 free ebook titles
- ipl2 - the Internet Public Library
- The Herzog August Library in Wolfenbüttel (HAB) -- Renaissance Books
- University of Illinois at Urbana-Champaign
- Project Gutenberg 46,488 free ebooks
- Trove.nla.gov.au - The National Library of Australia
- The Library of Congress, see particularly the Rare Book and Special Collections Reading Room
- The Hathi Trust - 11,434,302 total volumes with 35% in the public domain
- New York Public Library Digital Collections
- University of Virginia Library
- California Digital Library
- Center for Research Libraries
- Cornell University Library, Windows on the Past
- Digital Library of Georgia
- Digital Public Library of America - 7,675,354 items
- Europeana - Millions and millions of items
- Harvard University Library
- Internet History Sourcebooks Project
- Maryland Digital Cultural Heritage
- Metropolitan Museum of Art
- New Zealand Electronic Text Centre
- University of Texas at Austin
- Open University
- Oxford Text Archive
- Pandora Archive
- Project Gutenberg Australia
- Rare Book Room
- Texas Digital Library
- UCLA Library Special Collections
- United States National Agricultural Library
- National Library of Medicine
- Universal Digital Library
- Digital Library Production Service
- University of Wisconsin Digital Collections
- World Wide Web Virtual Library
- The J. Willard Marriott Digital Library at the University of Utah
- Wisconsin Heritage Online
- Wisconsin Historical Society Digital Collection
- World Digital Library
- Duke University Libraries Digital Collections
- Digital Collections | Columbia University Libraries
- University of Washington Special Collections
- Mississippi State University Digital Collections
- Emory University's Manuscript, Archives, and Rare Book Library (MARBL)
What you need to learn from this list and my attempt is to search for a book by title in every case, copyright or no copyright, to see if a website has an ebook for your particular search. Even then, it would be a good idea to search in the larger websites to see if Google missed the ebook. At some point, it will become possible to assume that an ebook always exists and then the search will be to find it online in some sort of library and gain access to the book. Interlibrary loans will very likely become completely digitized.
The above list gives no idea as to how many digital libraries exist in languages other than English. The number I found seems endless. Listing digitized map and newspaper sites was nothing compared to the number digital books sites I began to discover. Try searching. You will soon see what I mean.