I've been in GN&C for 20 years. The best book by far for entry level astrydynamics is Bate Muller White, Fundamentals of Astrodynamics. It's a paperback and it costs about $8 used. The math is the real deal and worth taking a look at (it's not for regular people). It was written to prep Airforce Academy grads in the wake of Sputnik. but if you ignore that and read the introduction of the sections it's an extraordinary background and foundation.