By my lights, a "popular" book is one that you share with more than 2000 users (of your particular edition, not for the book overall). I currently own 24 such books; the most popular, Orwell's Nineteen Eighty-Four, is shared with 10,202 users, and the runners-up are books like The Odyssey, Crime and Punishment, Lord of the Flies and Lolita. Really, no surprises there.
The more interesting statistics are the "unpopular" books -- books you share with less than 20 people. I currently have 45 such books, and they break down, roughly, into categories:
COMIC BOOKS. Nothing shocking about this. While there's a good number of comics nerds on LibraryThing, there aren't that many who know about 100 Bullets, and there are even fewer who will admit to having bought Marvel Essentials: Captain America Vol. 3 or DC Showcase Presents the Teen Titans.
COOKBOOKS. Fair enough. A highly personal choice, of course, and not all that odd that no one would buy the same ones as you.
BOOKS ON WRITING: There are millions of writing books on LibraryThing, but with a few exceptions (Writer's Market and a few by well-known authors, hardly anyone gets the same ones.
BOOKS BY PEOPLE I KNOW: Sorry, Matt, Scott, and John.
BOOKS CRITICAL OF AMERICAN FOREIGN POLICY IN THE MIDDLE EAST: Um...gee.
BOOKS ON HIKING OPPORTUNITIES WITHIN 60 MILES OF THE SAN ANTONIO METRO AREA: Okay, this one, totally understandable.
BOOKS THAT I AM SAD TO SEE ARE SO WIDELY UNDERREAD: In particular, the novels of Susan Straight and Stanley Elkin, the short humor of Ellis Wiener and Kurt Anderson, and the stunning true-crime study Murder for Profit by William Bolitho do not have nearly the audience they deserve.
Finally, there is the saddest category of all: Books Which, If LibraryThing Is Any Indication, No One In The World Owns But Me. I currently have three such books: Don Simpson's Megaton Man, Volume 1, DC Showcase Presents the Justice League of America Volume 2, and The Time Out (London) Film Guide Volume 4. (I am also one of only two people in the world who own Ellis Wiener's Decade of the Year -- unconscionable because it's such a funny book and a huge influence on my humor-writing style, but probably a good thing because now only me and one other guy will know where I'm stealing all my jokes), and one of only two people in the world who own the Larousse Biographical Dictionary, which is especially dismaying since it's a perennial candidate as my very favorite reference book of all time.
Folks, don't try this at home. It will eat up so much of your time, like masturbation, and leave you feeling really depressed and isolated, like masturbation, but it features almost no pictures of naked people.