Caution:
Some of the above free books are designed to get you hooked on the series. His Majesty's Dragon and Assassin's Apprentice (mentioned above) I found to be very good ... and I then went and bought more of both series's. But I only buy one at a time, when I'm ready to read them. There are so many other free books that I can read a free book till I have time to download a paid book in a series!