Having just sat the last of these exams (PPLH) i can recommend reading (and more importantly), understanding, W.J.Wagtendonk's book called "Principles of Helicopter Flight"
I got mine on ebay for £8 but you can buy it new from loads of inet shops.
Recommended by my heli school.
I can only remember the last two papers i sat having quite a few heli related questions in, they were Flight Perfomance and planning and Aircraft gen/tech. (ok, POF) Helped me pass for sure...
I also used this site.
http://www.airquiz.com/
£20 but worth every penny. (NB.
no heli specific questions though) if you do use this site better to print them off then you can research the ones you get wrong!
Flight test Thursday!!!!
(weather permitting of course)