I have written a very long post about my thoughts on how I try to approach a new math topic and the various factors that influence the best strategy, but now I'm not sure about the question. Are we talking about reading any book, or just math books? Is mastery of the content the ultimate goal?
Learning material for a test in a fixed amount of time is a skill that can be developed and can be very useful, but that's a completely different idea than trying to master the content and be able to use it effectively in the future.
Yet another possibility is reading through a large amount of material in a very short period of time and trying to become familiar enough with the general ideas and terminology to be able to hold a basic conversation but not go in depth at all.
What is the question?