What are some good textbooks for astronomy and astrophysics? I know of fundamental astronomy and the principles of astrophysics. I would prefer it if it were good for self-study. Also, are there any books that are free?
My experience in mathematics is calculus and I am currently self-studying multivariable calculus. I also have some experience in differential equations.
Yes, that book is something between a popular-science book and a undergrad introdutory textbook. If you want something for the advanced undergrad or even graduate level it's too simple.