One of my favorite photography ebook sites is Craft & Vision. They provide, "exceptional photography education at irresistible prices." I think $5 for a beautiful, digital ebook full of instructions and samples is irresistible.
Check out their free book entitled "Craft & Vision: 11 Ways to Improve Your Photography." They are celebrating their 3rd birthday the next two days with a 33% discount off 10 or ebooks. I love reading the books on my iPad. I receive no benefit from sharing their site. I just want to pass on a great resource.