Fundamentals of Celestial Mechanics
(J.M.A. Danby). About $30. Second edition last printed in 1992.
Solar System Dynamics
(C.D. Murray and
S.F. Dermott). About $80. Published in 1999.
Excellent introdution to
planetary dynamics written in a modern style at a graduate student level.
The New Solar System (J.K. Beatty and A. Chaikin, Eds., 4th Edition). $10-$20 second hand.
Excellent qualitative introduction to planetary science.
Descriptive chapters are each written by experts in the field. Published in 2000, so somewhat out of date but a good value.
Other Good Celestial Mechanics Texts:
Orbital Motion (A.E. Roy). Fourth edition published in 2005.
Good General Solar System Information:
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