Garry Kasparov's, My Great Predecessors?
Chess Books for 2100+ players

any book by Mark Dvoretsky would be a good choice at this level. If he doesn't have Dvoretsky's Endgame manual, that would be my first choice.
http://www.amazon.com/Dvoretskys-Endgame-Manual-Mark-Dvoretsky/dp/1936490137

Chess Books for 2100+ players
"Don't exist" is the end of that sentence.
... ok there are a few. Dvoretsky comes to mind. Any book that tries to crush your soul with endless analysis would probably work too... I've been told Kasparov's predecessor series fits that description :p
Other than getting him a challenging book you could get him a fun/interesting one that also serves as a classic to complete a collection. e.g. a tournament book (Hasting 1895 or Zurich 1953) or a world championship match (Tal-Botvinnik 1960).

I liked "How Chess Games are Won and Lost" by Lars Bo Hansen, "Positional Chess Handbook" by Israel Gelfer, and "Endgame Strategy" by M.I.Shereshevsky. These all helped me get better after I was already a master.
"Masters of the Chessboard" by Richard Reti is a great book for chess history, is a fun read, and is very educational. I learned a lot from it, but can't honestly say that it helped me play better.

100 Endgames You Must Know - de la Villa
Secrets of Modern Chess Strategy - Watson
And maybe an opening book if you know his repertoire.
Mastering Chess Strategy by Hellsten
How To Play Endgames by Muller (this is kinda like endgame strategy by shereshevsky but offers more material, and is more advance).
Practical Endgame Play Beyond The Basics by Flear. This is a huge book about strategical endgames.

Other than getting him a challenging book you could get him a fun/interesting one that also serves as a classic to complete a collection. e.g. a tournament book (Hasting 1895 or Zurich 1953) or a world championship match (Tal-Botvinnik 1960).
Oh God no, don't get Hastings 1895! I could only make it about a quarter way through that mess...some amazingly lame-o games.
Tal-Botvinnik by Tal...that's the one to get.
Also if you can find Kasparov's books about his individual WC matches with Karpov. I used to have a couple of those and they were truly excellent.
I would also suggest the following books.
Kasparov on Modern Chess
Karpov's Strategic Wins Vol 1 and 2
Invisible Chess Moves
Fire on Board: Shirov's Best Game

Give him a yearly subscription to New in Chess and he should be happy all year :) You may throw New in Chess Yearbook and Informator too :)
As for books: try to fill gaps in his bookshelf with Kasparov's volumes (predecessors, Modern Chess and vs Karpov, on Kasparov).
There two interesting Tukmakov's book. One might be quite usefull Modern Preparation (sic)
I have a friend whose fide rating in 2189. I want to gift him a set of Chess books on his 22nd birthday which will be very very much helpful for him. But i dont know which books will be good. Please suggest.
He is a book worm and already has a good numbers of books.