I still carry with me perhaps the most useful trinket I've ever received at a professional conference. Some years ago, the conveners of the annual American Astronomical Society's Division for Planetary Sciences meeting handed out with the registration materials a business- card-sized “cheat sheet” chock full of handy facts and figures on the physical and orbital properties of the Sun and planets, including a useful list of physical constants and simple formulae for solar system astronomy Along with (I confess) my similarly sized versions of the periodic table and geologic timescale that I have tucked away in my wallet, these form some of my most useful and handy sources of reference information for quick answers to typical questions that arise in planetary science.
Two new books have recently been written that serve similar purposes, though each at a different and higher level, for students and others interested in planetary science. The first is the incredibly comprehensive and well-organized The Planetary Scientist's Companion by Katharina Lodders and Bruce Fegley Jr. The authors, active and well-respected planetary scientists themselves, have responded well to the need within the community for a single compendium of current and useful data, formulae, and pithy synopses of information on the geology, geochemistry, interiors, and (where appropriate) atmospheric compositions of the objects in our solar system.