[TeeDub]

InclinedPlane mentioned "Working Effectively with Legacy Code," and I'd just like to add that's one of the greatest software engineering books out there.

Feathers (the author) methodologically chose to copy / inspire some of his examples from code he's worked with as a mentor / developer (some of it from his time @ Thought Works w/ Beck, et al.)

The book is similar to Fowler's Refactoring in that it provides some core thought processes / ideology and then a long list of heuristics and techniques for revising/updating legacy code. (I find his definition of "legacy code" as any software without automated testing to be challenging and insightful, especially since it indicts most code written ever. ;)

If you get the chance, you could also consider reading "Clean Code", it doesn't have /as much/ "real world" source but is still excellent.