I'm working my way through the Princeton Companion to Mathematics too. I like it because each topic is written in small digestable chunks by different authors.

I'm working my way through the Princeton Companion to Mathematics too. I like it because each topic is written in small digestable chunks by different authors. The change in writing style and view points keeps it refreshing. Although it's a bit high level, it gives you a taste for a lot of different areas in mathematics -- kind of a mathematical buffet.