I’d second that, although I’m not so sure that age has all that much to do with resale value. It’s the demand/rarity ratio and the condition that determine the value. It’s the ratio that’s important, not absolute demand, as I’ve found with scholarly monographs, which can go for several hundred dollars on the resale market only few years after publication—very few people want them, but there are so few copies available for sale, with most sitting on the shelves of university libraries. The fact that a book was once in the library of some well-known person might add a little to the value, but not much.
I sold about two thirds of my books before moving to Canada and in the process had several conversations with the bookseller on what determines resale value. Not only must it be a first edition, but it must be from the original print run to have any significant value. I didn’t have any books of particular value. The few nineteenth-century books I owned weren’t worth much. But the most valuable book in my collection was one that I though utterly unremarkable and not very old, a single-volume, hardcopy of The Lord of the Rings. In the late seventies Houghton Mifflin put out a fancy printing of the book, very nice looking, and intended more for display than reading. I got it as a gift and it was kept in excellent condition. IIRC, it originally cost, in 1970s dollars, about $30. I sold it for $60.
You can check abebooks.com and other second-hand book sites to see what a particular book is going for. That’s no guarantee that your copy will fetch that price—there are other variables like the local market and whether the particular bookseller specializes in that type of book (sometimes the specialists know how to get more money for a book and will offer more).
If the famous person has a following, some kind of fan society, you could try to market the book to them. Memorabilia collectors will often pay far in excess of what the general market would. You can also put it on eBay with a high reserve price, so you retain it if there are no serious bidders. That’s often a good way to test if there is some kind of specialty market out there.