When I did the exams I was recommended to use a slim volume called something like "Human Factors for Pilots" (it's in a box somewhere so I can't check), which was fine. HPL is a very easy exam (indeed all of the exams are easy) but the book was reasonably interesting. The Thom books have a truly leaden style, but are useful enough. FFF's suggestion of using Cap 413 instead of any RT book is a good one. The RT training tapes are in my opinion not very good, but RT is easily learned in practice by, er...talking on the radio from lesson one.