Hi Nervy
I am an HAI grad and FAA & JAA CFII. As Charlie said, princiles of flight is very good, I would further recomend helicopter aerodynamics by R prouty (quite expensive, but the best) and for a starting place the FAA's own book "helicopter flying handbook" which is a much easier book for the beginer. It's like all government books ienot always right, but any answer from there can't be questioned by the examiner, cos it's approved material. You will find that if you read several books, you get slightly different answers to the same question. Personaly I wouldn't waste your pre departure time learning the ASA prepware, as the written tests can be learnt & taken over a long weekend and cheaper at HAI than in the UK. The writtens are easy, but that is not the emphasis here. It's all about the oral exam before the flight test. These can be lengthy and you do need to know your subject to be able to talk convincingly about it.
Hope that helps
Swampy