There is a lecture I give to many students one-on-one every semester when they get into trouble. It is about how to read a math book, i.e. basic study skills. I recommend it for all my students. How to read a math book.
Something well worth looking at is the Mathematics Survival Guide.
This could explain a lot: Why Students Think They Understand - When They Don't.
This could explain a lot: Why Students Think They Understand - When They Don't. Local version in case the one on the web disappears.