There is one source here:
http://www.tolkienonline.com/docs/2825.html
And another here:
http://www.users.zetnet.co.uk/muss/topbooks.html
The first is an article about a 1997 poll of Waterstone bookstore chain's readers and a later, 1999 poll of Amazon.com customers, in which LOTR was at the top of both lists (much to the dismay of the literary critics).
The second is a poll currently underway of the top books of the 20th century. (There is even a form to fill out to submit your own top ten votes.) Currently, LOTR is at the #1 spot.
Hope these help! [img]smilies/smile.gif[/img]