It depends on why the book is hard. Sometimes it's just sloppy thinking and writing; sometimes it is an inherently hard subject. It isn't always clear when s

It depends on why the book is hard. Sometimes it's just sloppy thinking and writing; sometimes it is an inherently hard subject. It isn't always clear when starting which it is so it is useful, if you start having trouble to compare the one you are reading with another on the same subject. For reading that is inherently difficult, Mortimer Adler and Charles Van Doren give some very helpful information in How to Read a Book. Unfortunately it is not an easy read itself. Another book I found helpful was A Time to Learn by Phillip Bandt, Naomi Meara, and Lyle Schmidt. It has some really good ideas on diagramming complex work to help you understand them, especially how their ideas fit together.