[jansimonek]

I found the book Growing Object-Oriented Software Guided by Tests to be very inspiring. The book demonstrates how to evolve the code architecture, how and when to add new concepts and abstractions, how to listen to the tests, when to refactor etc. The tech in the examples is dated, but the gist of the book is very relevant.

Another classics are: - Working effectively with legacy code - Refactoring: Improving the Design of Existing Code

All of these books have a lot of depth to them