Although I would not profess to be technically strong when it comes to flying, I found that "Understanding Gliding" provides an excellent foundation for any budding pilot. derek write very well and it is a fairly easy read in a couple of sessions. Having read both the BGA Instructor Manual and this text, it is fairly clear that the former draws much of its content from Derek's work and the two combined are essential reading.
The only element I struggled with was visualising some of the more complex elements of the theory. The diagrams are helpful but the three dimensional nature of aerodynamics are not easy to illustrate. However, the benefit is that you are left with plenty of potential questions to pose to long suffering instructors!
This book should be in every glider pilots library.