Linear algebra, the study of vectors and vector spaces, has many incredibly interesting applications including mathematical physics, modern algebra and coding theory. Unlike more vague and new fields of Mathematics, our understanding of linear algebra is pretty air tight; you can make really satisfying progress in this topic and won't find yourself feeling like you've got gaping holes in your knowledge. Wanting to go into AI, machine learning or computing? This is the one for you.
Intro to Linear Algebra Series by 3Blue1Brown - this is a several part series so make sure you go through the different videos in order!
The author of Linear Algebra Done Right made this INCREDIBLE video series to go along with the book - a great combo for anyone hoping for a good understanding of undergraduate Linear Algebra.
Linear Algebra I: Linear Equations (This is a series of collected course materials from an archived course)
Differential Equations: Linear Algebra and NxN Systems of Differential Equations (For more advanced mathematicians who have learned the basics of Linear Algrebra)