For readers, programmers, and engineers stopping in here asking "ok what can I do to better myself here?"
I really recommend "Working Effectively with Legacy Code" By Michael C. Feathers. It is a top notch read, and helps newcomers (and vets alike) approach these legacy systems.
Marianne Bellotti’s “Burn It With Fire: Manage Aging Computer Systems (and Future Proof Modern Ones)” is another good book about working with legacy systems, though its focus is more about managing system evolution and modernization than specific code techniques.
I really recommend "Working Effectively with Legacy Code" By Michael C. Feathers. It is a top notch read, and helps newcomers (and vets alike) approach these legacy systems.