Great question — answering as the UphillTech team:
Amazon have a link checker tool on their associates website which will help determine if your links are valid, use that for testing. As far as the books go, you need to read the Amazon Associates Agreement and decide for yourself if you're breaking the TOS.
Full answer by toomanyairmiles: https://webmasters.stackexchange.com/a/30526 (CC BY-SA 3.0).
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