It depends on the giver and the recipient. A very wealthy person can give a set of seforim, while a person of lesser means can give an individual but still meaningful sefer. Seforim are less appropriate coming from a person who never looks at them, or going to a boy who will never look at them. Coming from a chosuvah talmid chochom, it is seen as a personal recommendation and endorsement for the bar mitzvah boy to read that particular sefer.
If you feel that the bochur will use the sefer, (even if not for a few years) a sefer is a good idea. If he is a lamdun, a less common but still heard of sefer is okay. If he is less knowledgable, more modern, or non-scholarly, an English sefer or even a book might be more useful. Some ideas might include:
– Rabbi Ribiat’s 39 Melochos 4-volume set (useful but not cheap)
– Rabbi Bodner’s Brochos Guide
– Any new Biography
– Amazing Facts (book 4? just came out)
– Artscroll Rashi (small, medium, large)
– Artscroll Shas (just kidding)
– Many new seforim and books come out every month. Anything that ‘just came out’ is safe to assume that he does not have it yet.
I am sure that any decent seforim store can assess you and your description of the recipient, and make a few recommendations.
Of course the bottom line is, anything you give ought to be appreciated, and it doesn’t matter that much what you give.