If you want to save money, look on eBay - but buy a new air law book, because that's changing all the time.
Speaking as somebody who flies microlights as well as light aircraft and has done both courses in full, it's a source of wonderment to me that Trevor Thom takes 4 volumes and about £80 to cover what Brian Cosgrove does in 1 volume at about £16 with better pictures and much more readable text.
Oh yes, and don't buy CAP413, you can download it for free from the AIS website if you've got access to a laser printer to print it out, since it's reasonably long.
P