Michael Feathers working with legacy code book has been an absolute tome for working with old code. So many good ideas on how to tame code and find bugs and slowly improve the quality of it and its testability.
I've been thinking of using this as an office book club reading. We have some very important systems that have not been sustained, have no or very outdated documentation, and essentially no tests (or none of significant value, a single, fragile end-to-end test barely counts as a test when it can fail just for looking at it sideways on a Tuesday in May).
It's a very good book and more people should read it. Some great insights on how to get control of an existing system.