Erwin,
Here are some examples of general purposes:
"What information in this book is relevant to me?"
"What are the main ideas of this book?"
"What has this book to offer for me?"
"My purpose is to pass this exam and read this book for it. Which information/ideas/knowledge/.../ in this book are essential in passing the exam?"
"How can I benefit from this book?"
...etc. I hope you get the idea. Since you are familiar with NLP you can easily expand from this. Good luck!