It is usually appropriate for textbooks and assignments where we have difficulty forming a solid purpose. It also helpful if you find the subject difficult and just don't know where to start. By going though the whole book you can select the easy spots.
The 20 to 30 minute thing applies to any book and for some that just becomes a post view. When you superread you. When you superread you consciously read very little, when you rapid read you consciously read more, rapid reading can be as slow as 70 words a minute in a section where you are doing analytical reading. When you correctly approach reading you will have viewed the pages a couple of more times before you decide a section is worthy of analytical reading.
No, 20 minutes through a book from front to back is usually not just rapid reading.
for those 5 to 10 minutes a clear purpose as to why you are even carrying that book and what question you particularly want answered, go into the book for that answer. Unlike passive reading you don't let the author ramble when you ask a question.
It's a bit like asking where the nearest convenience is and being told the history of it's making when you just want to know where the heck it is, the history can wait 
Alex
