The Goal was an interesting read after having read The Phoenix Project. It's what The Phoenix Project was based on. It's about the Theory of Constraints and in "The Goal" they're trying to save a manufacturing plant. (First edition is from 1984. They're up to fourth edition.)
I personally liked The Goal much more than The Phoenix project. That could be because I read it very early in my career, and also maybe because when I read the latter book, I was already following a few of the recommendations mentioned there.
