Ask HN: Good resources/Best practices about refactoring

Hi guys, i need to refactoring a 4 yo CMS (PHP Language) composed by a "monolithic" spaghetti code (5% oop & 95% functional/spaghetti).

Is there any good book about refactoring? Tips?

I know it's a hard task but i need to do it because, otherwise, it will be an suicidal CMS.

Thx!