For a single quarter basic introductory course, I recommend J. Matousek and J. Nesetril, Invitation to Discrete Mathematics, Oxford Univ Press, 1998. I have taught from it several times - it is well written, clear, inexpensive, and fun on occasion. It also has the advantage of being not overly ambitious in scope (compared to van Lint & Wilson, for example, excellent otherwise for a longer course and more advanced students), while still having a good selection of topics.