I used to gave a widget for this (js based, not WordPress), but it’s long since broke. I’d refresh it but I don’t want ony more j.js on this site than necessary.
Still, Librarything is still worth a look over.
Goodreads : Goodreads
This looks promising and has an extended api, but strangely what few WordPress widgets exist are few and far between and look hopelessly outdated. sadface
Anobii : Anobii
Has apps for iPhone and Android, and a developers page, but nothing out there for WordPress.
Shelfari : [ shelfari .com ] now redirects to Goodreads
Just validating links and code at present, Page not otherwise revised.