I really find Bate, Mueller, and White's _Fundamentals of Astrodynamics_ to be a
highly useful introductory book. Aside from including some non-SI units (Mars
again!), it seems quite simple (though not trivial). That is, of course, if
you've reviewed your vector calculus identities...:) But it has been great for
me to review/patch my own knowledge of orbits. For a beginner? Not bad if she's
had Junior-level or first year grad school mechanics. Kind of a morass for a
non-tech educated person at any level.

Physics and Astronomy