'Working Effectively With Legacy Code' by Mike Feathers is an excellent read for tackling systems like this. Basically a series of down-in-the-trenches strategi

'Working Effectively With Legacy Code' by Mike Feathers is an excellent read for tackling systems like this. Basically a series of down-in-the-trenches strategies for getting legacy code under unit tests - even if you're not sure what it's supposed to do - so you can start refactoring with a safety net.