Your purpose becomes like a thirst for knowledge. Just to desire something out of a book for me is wussie. It has to be significant if I'm going to invest time in it.
If I see a book titled "Use More of Your Mind" my question is yeah, right how? What tricks do you have that I don't already know? How do you substantiate that and how easy or complicated is it for me to apply? Do you expect me to stand on my head and wiggle my ears my ears to master how to remember a shopping list? How long does it take to learn?
As you can see my purpose to learn how to use more of my mind is in agreement with the title of the book. My questions are based upon my personal line of experience and naturally colour my questions. Comparing what I already know to the text at hand. I'm giving the author the opportunity to show me what's new to me. Naturally I'm not going to spend a lot of time superreading and dipping stuff I'm familiar with. Those pages I'll probably gloss over with the photoreading speed but with a more focused vision to be aware of minor variations that may show me something that I had missed in the technique that I know but in reality I consider this the last 20% of the book. I want the 80% of the book that is new to me. Might be only one chapter so I aim to read that in the bookshop and save myself some $
When it comes down to it, purpose is really the answer to the question... "When you have information from this book; what are you going to do with it?"
"i desire a general understanding of the concepts in this book"
Why? What are you going to do with it?
If you know what you're going to do with the information you ask better questions, speed your way through to the answer and dump books that really don't serve the purpose and get moving to the ones that do.
School books... "Teacher says we have to read this". Okay so what's your purpose for reading that? To please the teacher? To understand what the teacher is explaining so that...?
Always, and I mean always find your own purpose.