- Kindle / Kindle Ereader App - Opens files that end with .azw (from Amazon), .mobi, .pdf, .txt, .prc.
- Nook / Nook Ereader App - Opens files that end with .epub, .pdf., and .png.
- Sony / Sony Ereader App - Opens files that end with .epub, .pdf, .png, and .txt.
- Apple iBooks App - Opens files that end with .epub and .pdf.
- Kobo - .epub, probably .pdf. You might also check to see if your Kobo reader will open .txt files or .png (graphic) files.
Even if you don't have a reader, you can enjoy these free books. Just go to Barnes & Noble or Amazon, and download their free application to allow you to read these books on Android, IPhone, or PC. You may also like to download the free Kobo reader, which is compatible with Nook.
One thing that you'll find out almost immediately is that you'll need some way to manage your ebooks, as well as converting some that you download (those not DRM-protected) from one format to one that's compatible with your ereader. The best program I've found so far is Calibre. Not only is it relatively easy to use, it will open books in whatever format you download them in, including some of the more arcane ones -- and, it's free.
If you'd prefer to do this on your own, here's one simple method to "open Pandora's box": simply type in $0.00 into your favorite search engine. That brings up 167 million hits - too many - but it does give you somewhere to start. A better search string is "free Nook books" (replace Nook with Kindle or other sources to search for your particular reader). This search string will bring up 78,000 hits under Bing, and 1.4 million using Google. I'll make it even easier, and provide links to some of the best sites on the web.
The first place you'll want to check for free Nook books is with Barnes & Noble. B&N doesn't list how many free books they have, but it's a bunch! WARNING: Open each book in a new tab and make sure it's the whole book, and not just a "free preview". You'll also be able to determine if it's a book or just a short story (the query won't separate them). B&N says they have 1850395 free books, but a large number of those are previews or short stories. There are plenty of different books available, in multiple genres.
Another excellent source is Reading Fanatic. This site requires you to register, but it's free. They also supply a free toolbar that you can use to sign in or search their site.
You might also want to try BookBub. This one works a little differently: you have to register (enter your email address) in order to get their daily listings of books, or you can try browsing their recent listings. BookBub doesn't sell or distribute books themselves, but aggregates the offerings of other sites that do. Please be careful visiting these sites, as some of what's available won't be free.
One of the best places to find free ebooks is Project Gutenberg. Project Gutenberg has as its primary goal placing all public domain works online in multiple formats. Don't think these are just books from the 1800s by unknown authors! Some of the works listed by Project Gutenberg were originally published in the 1950's and 1960's. There are plenty of classics, also - books from such authors as Alexander Dumas, H.G. Wells, F. Scott Fitzgerald, Ralph Waldo Emerson, Willa Cather, Edgar Rice Burroughs, William Shakespeare, Jack London, and hundreds of others. Definitely worth your time! Make sure the copy you download is compatible with your reader device. Project Gutenberg usually offers more than one format, but not always.
Another excellent source is Many Books. This is another site where you have to be careful, checking the format the books are available in, and whether they're books or short stories. They don't usually list previews.
My second-most favorite site (after Project Gutenberg) is Feedbooks. They have several thousand free books, as well as books for sale. Make sure the price is "$0.00" before you download. They have several built-in searches by genre, and you can also search by author.
Open Library is another site that requires you register, and books are only available to be borrowed. It functions more or less as an online library. A number of public libraries also offer ebooks that can be downloaded. You might wish to ask your local librarian if they participate, and how.
Here's another one: Bookyards. They have quite a few authors listed, but I haven't used them yet, so I can't guarantee how well they work.
GoodReads is another site that offers free ebooks that I personally haven't used. I have a good excuse -- I already have over 400 books downloaded, and have read maybe five or six of those. My personal "TBR" (To Be Read) list grows with each passing day.
You might also want to check out Smashwords . Most of the books on Smashwords are from newer authors, but there are a few titles by best-sellers. I hope to have my books listed here in the future, but not for free.
Kobo also has a listing of free books compatible with its device (which usually means it's also compatible with Nook) at this location.
If you're a history or political science buff as I am, the Federalist Papers has a large list of books that can be downloaded, mostly in .pdf format.
Here are a few other sources. I haven't done much with most of them, simply because there is far too much available from the ones I've previously listed that I still need to download and read. These are listed more or less as I grab them.
Sony eBookstore Free ebooks
Obooko (British site)
Ebook 3000 Free books, including textbooks.
Ebooks to Go
Best eBooks World Many non-fiction, some fiction and poetry.
eBook Mall Free books. They have others for sale.
There are also lists you can join to get a daily offering of free ebooks and bargains. Two of these are listed below. If anyone has any additional suggestions, please feel free to post them in comments.
Pixel of Ink
eReader News Today
I wasn't aware myself what all was available when I first started this. Since then, I've found quite a few sites that, while they don't always provide free ebooks, they do provide some very useful information ABOUT ebooks, and links to more. Here are a few discovered with a search for free ebooks:
Freebytes page on free ebooks.
This is something else I hadn't seen before, but worth taking a few minutes to preview. This is a discussion net on free ebooks. Check it out if you're interested.
Free e-Book Download Net
I could probably continue this list indefinitely (most queries for "free ebooks" bring up between 35 million and 40 million hits), but that would be exhausting, and some of the links aren't really for "free" books. If you do such a query, read the page very carefully, as some of them only allow you to "preview" a particular book, or to download an exerpt (usually a couple of chapters). Others require you to "subscribe", and that may open you up to a constant flow of spam email.
In the meantime, enjoy!