There's actually no books needed for w101! The OU do not send any books or materials etc because everything you need is online, on the OU website (you can request to be sent paper print outs of the online material, but it's nothing different to what is on the module website and most people don't do this)
A lot of people buy 'Letters to a law student' and OU also say it's a good read, but 100% not needed for the course :-)