I remember when Martin Fowler's Patterns of Enterprise Application Architecture came out back in 2002. I constantly heard good things, but never got around to reading it. Finally, I buckled down and went through it, and I'm glad I did.
Perhaps the biggest thing I liked about the book was the common vocabulary. Whenever I look at popular community projects (such as Enterprise Library, CLSA, or .NetTiers), or just read star bloggers, ones keeps hearing about all these pattern names (ActiveRecord, Gateway. Lazy Load, Repository, Registry, Service Layer, etc...). While gradually you pick them up, it's convenient just injecting them into your brain all at once.
I also thought his chapters on concurrency were excellent, especially how he explains the difference between an optimistic lock and pessimistic lock. (My simplified interpretation is that an optimistic lock is "optimistic" in that it assumes conflicts are either rare, or easy to resolve, and hence checks for conflicts at the end of a transaction. On the other hand, a pessimistic lock is "pessimistic" in that it assumes conflicts are either common, or very expensive to resolve, and hence prevents the conflict from ever occurring by locking at the beginning of a transaction).
He's also very "no-holds-barred" for doing things the best way. For example in the Metedata Mapping pattern he emphasizes using code generation or reflection - two concepts that for some reason many developers seem reluctant to use.
Lastly, reading a solid book like this just helps you think more about enterprise patterns as you go through your daily coding, and that's a valuable thing.