NLP is the abbreviation of Neuro-Linguistic Programming.
There are two books I always recommend as starters:
1) Trance-Formations by Richard Bandler and John Grinder 1981
2) Frogs Into Princes by Richard Bandler and John Grinder 1979
Both are now hard to buy, but easily downloadable in PDF form from the Internet.
Of the two, Trance Formations mainly deals with the Hypnosis side of NLP, while Frogs Into Princes mainly goes into the internal self-transformation aspects of it. But they both overlap, and both should be read to get a complete picture.
These two books thoroughly explain the principles and concepts right down to the nuts and bolts of it - but they are poorly written, and don’t give practical real-world guidance on how to actually do anything with it... (They were both conceptual, written for experienced therapists, and gave very little practical guidance on how to practically apply the concepts to yourself or others.)
Nevertheless, I would say they are both mandatory reading if you want to truly understand the subject.
Only get the early versions - Richard Bandler has recently released a newer version of the Trance Formations book and it is useless...
The next book I could find when I first started learning, was the unfortunately titled “Be The Person You Want To Be” By John Emerick, which actually turned out to be a well written guide to the practical implementation of the principles described in the first two books.
There are other good books from that period by folks like: Tad James, Robert Dilts, Joseph O’Conner.
Their books both expand upon the theory of NLP in various directions, and also give practical guidance on how to use it. They are also much better written than the Bandler books.
Most of the modern material I have seen is very superficial, and is often written by people who appear to have never seen the core material.
Hope this helps!