Did you know that Goodreads has a quotations database with tens of thousands of quotations and about 40,000 tags? http://www.goodreads.com/quotes Some of them are even (loosely) sourced, although I don't know how because there's no source field in the new quote form: http://www.goodreads.com/quotes/new . I'm wondering if this makes my idea of a Wikiquote profile on Goodreads (http://www.goodreads.com/itwikiquote ) useless, and if Goodreads users may be interested in reading (and editing) Wikiquote. In that case, you may try to create an English Wikiquote profile: you can even import books without ISBN, all listed in a CSV spreadsheet with author and title (or even title only), and they'll look for matching books automatically.

Nemo