buying books for readers can be hit and miss, as some people are quite particular about the books that they read, making it hard to choose books on their behalf (especially if you are not a bookworm or interested in similar books yourself).
If you DO want to gift books, a theme might help make it easier to select them, e.g. booker prize nominees, banned books or books that made the BBC Big reads shortlist. www.bbc.co.uk/arts/bigread/top100.shtml
You can get beautiful display copies of certain books, e.g. clothbound penguin classics, if there is a book that you know is special to him.
If he buys paperback books, might he appreciate some quirky bookends to display?
If there is a particular TV series or film that he really likes, perhaps he would appreciate the books that inspired it? If he's of the old school technology persuasion, you could include the accompanying DVD.
Does he have a kindle? If not, might he appreciate one? (Worth doing some digging as many people prefer tree books). If he already has a kindle, does he have a page turner?
Might an audible subscription work so he can listen to books?
If he has a favourite author, is there a day trip you could go on connected to that author?