When you purchase a book from Amazon.com through one of the 'Buy this book' links, Amazon pays a small referral fee. The money doesn't go to me, however, but to the Python Software Foundation.
I spend a good deal of time programming in Python, an elegant and adaptable programming language. The Python Software Foundation is a non-profit organization that retains the copyright to Python, makes the language available as free software, and supports development by organizing conferences and administering grants.
Your book purchase, therefore, is contributed to a worthy cause.