I think the best first book to read on Buddhism would just be The Heart of Understanding by Thich Nhat Hanh. The Tibetan Book of Living and Dying is really good, but I would read that next. It's much longer, and although parts of it are really insightful, there are also large sections that are repetitive or esoteric. It's also about ideas and practices that are important to Tibetan buddhism, but that aren't central to most other forms of buddhism.

I know it's a completely different arena, but how do you feel about Hesse's Siddhartha?

I haven't read it, but it seems cool.