Ask HN: Good resources about legacy code?

Hello HN: I got offered my first consultant job for a company with a really old&bad (no documentation, spaghetti, monolithic...) PHP codebase. Most parts of the codebase is working fine in production but some parts have to be replaced. Can you recommend any good books/papers/websites on how to get started? i don't need language-specific material. i need methodic/abstract advise.