In addition to Working Effectively with Legacy Code, Re-Engineering Legacy Software[1] is pretty decent. It's really a lot of the same recommendations; put a test harness in place to guard against regressions, then start pulling out things and making them into more sane, SOLID components. For that aspect, you can look at things like the Uncle Bob Clean Code series, or my personal favorite Adaptive Code via C#[2] which despite the title is pretty general and all about writing code with the SOLID principles.

[1] https://www.manning.com/books/re-engineering-legacy-software

[2] http://amzn.to/2CfxK8w