You are probably best to ask the school which ones they use.
These were mine, but bear in mind my PPL(H) was in 2005/6.
I really enjoyed LEARNING TO FLY HELICOPTERS by R Randell Padfield but didn't find it very useful for the Helicopter specific written exams I had to do (I already had a PPL(A) that excused me most of them).
The recommended book for my PPL(H) was was written by W Wagtendonk, called Principles of Helicopter flight and seemed (to me) impenetrable. I gave up on it. It would probably appeal to someone with a more mathematical/engineering approach than me.
After I had my licence I was given a pre publication copy of Private Helicopter Pilot Studies by Phil Croucher, which I thought was very good and wished it had been available when I was a student.
And you should read Chickenhawk.