This book has been popular since it was first published. Basically it is a book dedicated to the discussion of heuristic, which literally means "serving to discovery" in Latin.
The reading experience may not be very comfortable for a modern reader, provided the author used a somewhat old-fashion kind of English. Another point that hampers the readers' enjoyment of this book may be the third section, "Dictionary". It is arranged according to the alphabetical order and usually one can see much redundancy in different subsections.
However, without much doubt, this book is the most classic book of its kind so far. A careful second-time reading can provide a much more clear picture of what this book is trying to address.
Personally speaking, I believe it is a very precious work that should be recommended to most math and many science majors. I also believe that writing some notes can help one understand the topic and develop his heuristic thinking better.