It's a great book especially for those who have never researched about memory. For me personally its a 3.7 because I have read a lot on memory and it was a bit repetitive,but for such a person it might be a 4.3, 4.5. The book refers to are memory ofcourse chapter 1 telling what are memory actually is and can do. It also tells you unbelievable facts. My favourite was that memory is unlimited and you only have to steer,practice and harness your gift. It refers to how we memorize things. Short term and long term memory etc. Chapter 2 is memory tricks with a lot of tips and techniques to remember names and dates , also looking on memory methods used by other people. Chapter 3 is basically why we forget and how not to forget. It's not even that legnthy with 140 pages including huge pictures.
This book was lent to me to read because of its tips to improve memory. By now it's a pretty old book, there were few tips I found useful but it was mainly a book I skimmed through and skipped pages of.