BEWARE - When it shows a price and then under the price it shows that it's free for Prime members IT IS ONLY FREE for prime members that have an actual Kindle device. If you are like me and read them on you PC then it is not free even if you are a Prime member.
All of the books listed are or were free. Some are only free for a day so you have to constantly be checking this thread or the Amazon kindle store to track them down. The way I do is to use the keyword photography on the Amazon.com Kindle store page.
I would then change the " Sort By " option to " Price: Low to High ". I check every morning & there are usually two books that are free for the day. In most cases, they'll go back to requiring payment the next day.