Librarything : Librarything
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and has an extended api, but strangely what few WordPress widgets exist are few and far between and look hopelessly outdated.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
Powered by Amazon. Doesn’t appear to be a an api or developers option.