The author is Don Miguel Ruiz, and he's written a few follow-ups. 'The Mastery of Love' feels like a direct companion piece, diving into relationships with that same Toltec wisdom lens. I found it a bit repetitive if you've just finished 'Four Agreements', but the focus on emotional wounds made it stick with me.
He also has 'The Voice of Knowledge', which tackles that internal critic. It's more abstract, honestly. His son Don Jose Ruiz co-wrote 'The Fifth Agreement' too, which adds 'Be skeptical, but learn to listen'. Some fans think it overcomplicates the original four, but it's worth a skim from the library.
Beyond those, there's 'The Toltec Art of Life and Death', but it's a novel-ish spiritual journey, not a straightforward guide. His material gets pretty niche after the main books.