require 'amazon/search'
$AWS_KEY = 'Your AWS key goes here' # See below.
def price_books(keyword)
  req = Amazon::Search::Request.new($AWS_KEY)
  req.keyword_search(keyword, 'books', Amazon::Search::LIGHT) do |product| 
    newp = product.our_price || 'Not available'
    usedp = product.used_price || 'not available'
    puts "#{product.product_name}: #{newp} new, #{usedp} used."
  end
end
price_books('ruby cookbook')

Ruby Cookbook (Cookbooks (O'Reilly)): $31.49 new, not available used.
Rails Cookbook (Cookbooks (O'Reilly)): $25.19 new, not available used.
Ruby Ann's Down Home Trailer Park Cookbook: $10.85 new, $3.54 used.
Ruby's Low-Fat Soul-Food Cookbook: Not available new, $12.43 used.
...

Error! (Exception?) Here's stdout:
Amazon::Search::Request::HTTPError: HTTP response code 500
	from /usr/local/lib/site_ruby/1.8/amazon/search.rb:939:in `get_page'
	from /usr/local/lib/site_ruby/1.8/amazon/search.rb:961:in `search'
	from /usr/local/lib/site_ruby/1.8/amazon/search.rb:697:in `keyword_search'
	from (irb):5:in `price_books'
	from (irb):11

http://xml.amazon.com/onca/xml3?KeywordSearch=ruby+cookbook&mode=books...

...