Linear Algebra Done Right by Sheldon Axler for the following reasons:

- I was revisiting a topic in greater depth, which is a common theme in university-level math courses.

- It is a rigorous book, written in the style of definition, proposition, theorem, etc.

- It was the first math book where the exercises don't just reinforce what you learned in the chapter, but teach you new material (another common theme in advanced math textbooks).

- Linear Algebra is arguably the most important math subject these days.