This is the first year we have ereaders in the house. As I was searching for books for Kindle, I began to find some of the books we wanted to use with our curriculum for free. It actually felt kind of random until I started doing some more searching and realized that there is a way to increase the likelihood of finding free books.
The best known site for free public domain children's books is The Baldwin Project. You can use this site to read the books on your computer or print them out. You can search by title, author, or genre.
The other two sites I use for free books is Google books and Amazon Kindle. I have both of these apps loaded on my iPad. You can download their apps for different devices (android, iPod, iPad, and computer). For both of these sites it is pretty important that you use a generic search. Also just because a book isn't available on one site doesn't mean it won't be available on the other.
If you search for Famous Men of the Middle Ages you will find it on Kindle for either $2.99 or $.99 but if you search on Google books it is available for free.
On Amazon Kindle I have had the most luck not putting the word "free" in my search. In google it doesn't seem to affect your search and usually if you use the word free at the end of the keyword search (Famous Men of the Middle Ages free) the first book that comes up (if it is available) will be the free version. So when you use Kindle don't use free, but use free when you search on Google books.
Here is a link to Amazon's Free eBook List.
I have not searched for books for the Nook but one tip I read said to type in the subject you are searching for plus 0.00...i.e. King Arthur 0.00.
Here is one last link for free ebooks Happy Hearts Homeschooling Library. This is a new blog that I found from a link on WTM. So far I love what I am finding. She has a diverse list of books and has a great category system linked. On her side bar she has a list of places you can find free ebooks and also tips for printing and searching her site.
One thing to remember is that many of these books are not revised editions. Some of the things in them will not be politically correct and will reflect the cultures and mores from the time they were written. So be aware of this and be prepared to address any of these issues that arise from the reading.
Most of the classics will be available in some form for free (Charlotte Bronte, Stevenson, Dickens, etc). Also some "classic" textbooks will be available. I have a long list of language arts curriculum available on Google books.
The authors John Henry Haaran, G.A Henty, Samual B. Harding, and H.E. Guerber (all historians) have numerous books available and I feel like you could create a solid base for your historical studies using these authors. I don't believe that you could use these books alone. You need some more modern works. These books are not full of color photographs so they may not be as "entertaining". My plan is to use these along side other more current works so that the program is more well rounded. I love the Usborne books for their photos and the way they bring history to life. I also love the narratives in Story of the World for younger children. I believe that adding these books in gives you a depth that isn't found in current spine/core/textbooks.