Linear Algebra is a basis for so much in Machine Learning, and this is a free, open textbook that takes the beginner through the basics. I've not read it myself - I'm not the intended audience - but I've seen several reviews from people whose opinions I trust, and it seems to be well respected.
Honestly, I felt like linear algebra was the low point in my math career-- even behind axiomatic set theory. I still have nightmares about determining whether a set of vectors is linearly independent.
Linear algebra is frequently taught very poorly. I think most university courses in the subject are kind of watered down and reduced to algorithms you can memorize -- which, ironically, makes them tough going for the strong students.
