Author Mark Watson has generously made some of his ebooks available free of charge. See the books section of his website:
If you wish to pay for them, there is a choose-your-price (within certain limits) over at Leanpub, where you can also see the books’ tables of contents and blurbs.
Terry Freedman qualified as a teacher in 1975, has written for educational publications since 1989, and has published this website since 1995.