If you want something that specifically covers the (UK) PPL(H) syllabus, check out Phil Croucher's PPL(H) book - I forget exactly what it's called, but I believe Transair stock it. Or you could PM him directly and ask (he posts here as Paco).
If you struggle with Wagtendonk a bit, try Principles of Helicopter Flight by Jean-Pierre Harrison. It's a bit hard to find, as I believe it's out of print, but it's far more readable than Wagtendonk, and covers some stuff that he doesn't.