Suggest you try "Budgie The Little Helicopter" by......only joking!
The best and most easily undestood reference book on the subject is "Helicopter Aerodynamics" by RW Prouty. This was published in the mid-eighties so may be out of print.
Failing that, "More Helicopter Aerodynamics" and "Even More Helicopter Aerodynamics" by the same author, are fine in themselves but may sail a bit to close to academia (ie a maths degree)to answer your question simply.
Best of luck.