"Which are the best books on NLP?" and "Which is the best book on NLP?" are two of the most frequently questions asked by visitors to this site.  And if you are now one of those people, here is the answer to your question:

There isn't one.

That is to say, there isn't any one book, nor even just small set of books which would be the best choice for everyone, so there isn't just one answer to either question.
Mainly because it depends what it is you want to know about NLP - and what you already know.

NLP itself is simply a form of modeling, but it is accompanied by numerous related methods and techniques, which is probably what most people think of when they think of NLP.  (I've taken to referring to this wider topic as the FoNLP (field of NLP).)
And although NLP is just a specific form of modeling, the techniques that go with it can be used in many different contexts - therapy, education, business and social communications, and so on.  There is therefore no "best" book for the FoNLP, because no book covers all of those different subjects in as much detail as some of the more specialised books cover specific topics.

For example, a majority of NLP books deal with what are called "meta programs" to some extent.  But as far as I know, only Shelle Rose Charvet has written an entire book on the subject - Words That Change Minds - and even then, she only deals with thirteen out of the 29 meta programs covered (in a lot less detail) in Woodsmall and James' Time Line Therapy and the Basis of Personality.

So, as well as this page I've created a list that shows reviews of introductory level NLP-related books, divided up according to their main focus - business, education, communication, general and so on.
I hope this will help you to find the book(s) you are looking for.
 
Introductions to NLP
 
For the best amongst more advanced NLP books I can only suggest that you look through the highest rated books in the overal list of reviews - always remembering that the reviews represent my opinions, with which people have been known to disagree, occasionally