Free books for Kindle? There's tons in the public domain. I grabbed a few for the Kindle App on my phone before realising that a 3" screen (or however big it is) isn't the best place to read em.
So we're talking Frankenstein, Aesop's Fables, Sleepy Hollow, Grimm's Fairy Tales, Dorian Gray, Alice in Wonderland.. The old classics. Back when vampires didn't sparkle but lived in Whitby.
http://www.amazon.co.uk/Legend-Sleepy-Hollow-ebook/dp/B000JQUJHO/
http://www.amazon.co.uk/Frankenstein-ebook/dp/B000JQUZCI/
http://www.amazon.co.uk/Picture-Dorian-Gray-ebook/dp/B000JQU4TW/
http://www.amazon.co.uk/Dracula-ebook/dp/B000JQUBRM/
http://www.amazon.co.uk/Grimms-Fairy-Stories-ebook/dp/B000JML1QG/
http://www.amazon.co.uk/Adventures-Robin-Hood-ebook/dp/B000JQV5GI/
http://www.amazon.co.uk/Gullivers-Travels-ebook/dp/B000JQUZ3W/
http://www.amazon.co.uk/The-Jungle-Book-ebook/dp/B000JQU7JO/
http://www.amazon.co.uk/Alices-Adventures-in-Wonderland-ebook/dp/B000JQV3QA/
http://www.amazon.co.uk/Treasure-Island-ebook/dp/B000JML7EC/
http://www.amazon.co.uk/Oliver-Twist-ebook/dp/B000JQUT8S/
These are on the UK site but you can more than likely find them on the US site too.
Oh there's stuff like origin of species, works of Da Vinci, Theory of Relativity, History of England, etc.
And as mentioned these are all free to anyone with the Kindle App, available for Android, iPhone, iPod, iPad, and PC.