You don’t have to limit yourself to the author they love. Quite the contrary, it’s a good opportunity to expand the person’s selection.

Ask them what they love about the author. Not only does that make you a more interested friend, it gives you valuable information when looking for a gift. Ask online (or the sales person at the bookshop) “what do I get for someone who likes author X for reasons Y” and pick what resonates.

As (another) bonus, it shows the person you care enough to do your research and not just get what would’ve been on their wish list anyway.

My parents did me a great favor when one year they got me a book on Richard Feynman. I was very much into his stories and trying to understand his perspective on physics for a long long time after.

That's totally fair and this is a hard one I'll admit. I've done it for when I know there is a new book coming out that they might not have on their radar or I will ask friends/family of the person if they have the book. Not a perfect system and probably not the best example.

Bookstores often let you return and credit it to another purchase. Good reason to avoid online I suppose, unless they can streamline that.