Try this as an idea:
Keep an eye out for any new books that are published relevant to your niche. Use that as a prize for some sort of competition. You can buy it or get someone to donate it.
Use the competition to create a buzz about the book. Include your Amazon (or other) affiliate links for others to buy the book.
Even better, contact the author and ask them to judge the competition. This will create more buzz about the book – more sales through your affiliate link and the author will love the buzz.
Even better, the author is probably a prominent person in the niche, so you have now got them as a member on your forum (they have to post who won the competition!) and hopefully they will become a useful member.
Its all part of the plan!
Also keep in mind if using Amazon.com, that they are based in the USA. I serve up the affiliate links from Amazon.com, Amazon.ca, and amazon.co.uk, amazon.de and amazon.fr via the geotargeting of OpenX.
For more tips on how to use the Amazon Associates programs to further monetize a forum, see Monetize your Forum with Amazon.