For books published by the Pragmatic Bookshelf, you should link directly to the publisher, in my opinion. PragProg.com sells DRM-free ebooks (in pdf, mobi, and epub) for cheaper than Amazon. They are a great value and I bet the PragProg folks keep more money this way, which is a good thing.
For example http://www.hackerbooks.com/book/rails-for-net-developers-fac...
Should link to http://pragprog.com/titles/cerailn/rails-for-net-developers instead of Amazon.com.