In more advanced undergraduate math, PDF lecture notes written by professors but not published in book form often contain excellent explanations and proofs that

In more advanced undergraduate math, PDF lecture notes written by professors but not published in book form often contain excellent explanations and proofs that are not available in books. Also, making undergraduate buy expensive text books is more of a thing in first year American classes. Look at the lecture notes at Oxford for example. https://courses.maths.ox.ac.uk/ https://courses.maths.ox.ac.uk/