I’m addicted to books about Buddhism; I have somewhere between two and three hundred of them. In my estimation one of the very best is Foundations of Buddhism, by Rupert Gethin.

It’s a relatively short book that nonetheless manages to provide a comprehensive overview of the fundamentals of Buddhist doctrine and practice, as well as a study of the textual corpora and historical contexts. Gethin explains difficult concepts with lucid prose and makes excellent use of the latest scholarship (as of 1998) to summarize different theories regarding such controversies as, to give one example, how the Mahayana traditions arose and evolved.
Gethin does a terrific job, in particular, of clarifying concepts we are often hazy on. One thing you’ll find when you start reading works of Buddhist Studies in addition to Buddhist books is that often the points that we find confusing are points that the tradition itself is confused about. That is to say, there is usually a history of doctrinal conflict and evolution about which it is very helpful to know.
There are a number of detailed, five-star raves on Amazon.com, a few of which make a better case for the book better than I can, so suffice it to say that this is one of the few books on Buddhism I would unreservedly recommend to anyone with more than a passing interest in the subject.