A nifty book collection-cum-discussion site is a reality with Shelfari. As proposed by Chirol at Coming Anarchy (hat tip) Shelfari looks likely to get swallowed up or copied by some of the large webbased booksellers like Amazon. Amazon already has both "I own it" including personal ratings plus the possibility of writing reviews -- turning this information into a social site would be easy.
One way to enhance the usefulness of Shelfari as it is would be to develop the possibility to have identical-but-parallel, concurrent discussions on Shelfari and (between) one's blog(s) at the same time. Another useful thing would be the ability to import exactly the Amazon "I own it!" list to Shelfari.
UPDATE: Actually, Shelfari does have some kind of coop with Amazon so the next step would be logical.