Reason for recommendation:
The one and only origami book devoted entirely to the theory of representational origami design. A must-read for anyone who wants to learn how to design.
It is impossible to write origami book review articles without mentioning this one, for a very simple reason. Although we do have many books mentioning fragments of design theories here and there, this is the ONLY book that is entirely about the design theories (if there is another one, that can only be the first edition of this book).
Luckily, this book is also written by one of the greatest origami masters of all time, Dr. Robert J. Lang. I can confidently claim that Lang has the best overall skill and knowledge about origami than anybody else: not only he is one of the greatest designers and folders, he also knows everything from the toughest origami math to the latest applications in engineering. He is also a great writer; I'm sure that anybody who reads this book would enjoy his wit and literary grace.
This is a very heavy book with more than 700 pages, but trust me, it's not as scary as it looks. Targeting the widest possible range of readers, Lang will not bombard you with deep mathematics in this book, and instead, he will break down the ideas behind the design theories progressively through lots of examples and figures, making them easy for anyone to grasp. From simply modifying traditional bases to creating crease patterns of the highest complexity, this book covers them all. If you ever wonder how exactly origami artists came up with all those seemly impossibly complex designs from a single square without cuts, this book has all the answers you need.
And if you had read the first edition before, I will also strongly recommend you to get the second edition as well; I'm sure that you would not want to miss the new materials related to uniaxial box pleating and polygon packing, as well as the various CP challenges.
In short, if you want to create your own original designs, rather than just folding designs by others in awe, then you really should study this absolute masterpiece.