Depends what your math level is. Start with https://www.expii.com/ which is Po-Shen Loh's site and see where you are. If you can do the algebra and basic calculus problem sets there you could also try these specific ML background books https://sites.google.com/site/10715advancedmlintro2017f/read...

Expii is a great resource because it determines from your success solving problems if you need more practice to get a concept