Actually, not technically free as the authors are asking for a “pay what you can” donation, but that payment is totally separate from the actual download, so you can read the book and decide if you liked it and then make a choice about donating. The book in question is “How to create Stunning Digital Photography” by Tony Northrup and the free site is available here.
I’ve downloaded it and am currently reading it – so far I would say that is it a nicely put together book and I’ve already learned (or re-remembered perhaps) some interesting tips. I’m definitely going to be one of the people that donate. The book has five stars on Amazon from almost 2300 reviews, and is deservedly popular. So check it out!