Ask HN: What are some good resources for fixing technical debt?

I'd like to fix technical debt that we have while still be able to create new features.

If needed here are some details for our system: - Systems that should've been separate are all in a single huge monolith. They integrate with each other all over the place without any proper structure. - Core functions are mostly static functions and test coverage is really low. There are no unit tests but there are some integration tests. - +200k LOC