M. Feathers's book "Working Effectively With Legacy Code" is excellent. The focus is on programming techniques to decalcify a legacy codebase, but learning those techniques makes it a whole lot easier to reason about and prioritize work on such a system.
Used it for some small group communication when planning a bike race. It was a great alternative to 20-deep CC lists and Reply-All, and I think would have worked for our cycling club's internal mailing list if there had not been a cap of 50 members. It was nice to have a simple interface for managing list members (compared to, say, our web host-provided ezmlm lists), but its closing won't hinder our future event-related communication.