My Twitter book recommend project

I've been tweaking a book recommendation engine for twitter. The algorithm is based on tf-idf of words in a user's tweets. Relevant words are extracted then fed into the amazon api. Would appreciate feedback.

http://foxblot.com/

sample recommendations: http://foxblot.com/pgraham http://foxblot.com/barakobama http://foxblot.com/fredwilson

the recommendations would get much better if I used phrases instead of words. All feedback appreciated.