Modeling has an excellent book that explains R/C design from A to Z.  It could be a textbook except it's so readable. 

R/C Model Aircraft Design by Andy Lennon.

It explains everything and does it simply.  There are some formulas, but after you've read it, you'll understand the few that're needed.  And they're simple ones.   Amazing book.   It's so good that Model Airplane News published it, undoubtedly because it is so understandable.

It's often sold in LHSs and is usually about $20.