People who love to read tend to gravitate to the same kinds of books. It's not that they aren't open to trying out something different, it's just that they know what they like.
If you are still looking for a gift for someone that you know loves to read, I'd suggest getting them a book that they wouldn't pick out for themselves. It doesn't need to be something drastically different than what they'd choose. Don't get a Regency romance for your Adrian Goldsworthy fan, but maybe something from Bernard Cornwell.
The most important thing is that the book you give should be one that you've enjoyed yourself or that has been given a good review by someone whose taste you trust. Why? Because if your bookish giftee receives a book in which they can see no redeeming value, they'll never trust your recommendation again.
Not sold on this idea yet? Think about your favorite books and authors, I bet you were introduced to at least one of them by a friend, family member, or colleague. No? well, bah humbug to you. Here's my final attempt at proof of concept: When I was visiting my parents for the Thanksgiving holiday I ran out of the reading material I brought from home. My parents' house is full of books so I had plenty of options, but I ended up starting a book that my father had set aside for me: Old Man's War by John Scalzi. He was reading one of the sequels and assured me that it was a great series. I would have never picked up this book on my own being as it was mass market scifi with an unappealing (to me) title. I liked it!