Level: Basic
This is a far more difficult question than you might expect. Judaism has no dogma, no formal set of beliefs that one must hold to be a Jew. In Judaism, actions are far more important than beliefs, although there is certainly a place for belief within Judaism.
As I said above, Judaism focuses more on actions than on beliefs, and books about Judaism tend to do the same. Most books emphasize holidays, practices and observances. The best summary of Jewish beliefs I've seen is Milton Steinberg's Basic Judaism. This book presents and contrasts the traditional and modern perspectives, and shows that we have more in common than many of us realize.