[gruffgirl16]

I'm reading Refactoring JavaScript by Evan Burchard. While you are focusing on a "java" codebase, this book has general tips and suggestions that are languange independent. It's quite good about going through various code patterns and how to refactor them.

[yesenadam]

Well, why not the original Refactoring, as it's more OOP-oriented, and actually has Java examples. Fowler's website, as has been mentioned already, has a lot of his refactoring-type material online. https://refactoring.com/catalog/