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This is a great book to sit down and read a few articles from (at least if you're into reading encyclopedias ;)), but you aren't going to learn much "actual math" from it.

Most research mathematicians don't have a clue what people outside of their field work on, so this is a great book if only because it lets to you talk to the guy down the hall :-) Use it to figure out what interests you mathematically, and then drill down to various subjects with more in-depth texts.

I just got it, and it's huge. Bigger than most dictionaries.

The articles are great. It's a useful resource to look up something you hear a reference to but haven't seen before, or to get more cultural exposure around something you're studying in a narrow context. I recommend it, though you'll need another text for a thorough, linear introduction to a field.