Two books spring to mind automatically. One is Darrol Stinton's "Flying Qualities and Flight Testing of the Aeroplane" which covers in outline much of the theory very well.
A second, which is rather offbeat but covers safety planning, teamwork, etc. is Vaughan Askew's "Flight Testing Homebuilt Aircraft". A less readable (but free, you should be able to download it on the net somewhere or other) is the FAA's flight test manual AC23-8.
For general pre-course prep, you also want to revisit your basic maths and aeronautics. Any good engineering maths book will do the former, and I've yet to see anything better than the RAF's AP3456 "Flying" for the latter.
If you're doing ETPS, I'd add in something equivalent to the RAF's ISS - get your report writing to predestined formats utterly up to scratch. If it's one of the other schools, this is still useful, but perhaps not quite so critical.
Finally, I'd say pick a couple of aeroplanes you know reasonably well, and get to know them really well - using the standard operating data. Being in the habit of learning your way quickly around the systems of an aircraft will do you a lot of good.
Hope this helps, best of luck with it.
G