Not really a "code" but you can earn gift certificates for minimal work. Go to
InsiderPages and click the link for the
Amazon promotion they're running. Do up to 30 reviews for Amazon GCs. You earn a $10 GC for each 10 reviews that you do, up to 30. Two people in the same household can do reviews, so you can technically earn $60 in free Amazon GCs.
There are some rules as to what you can review (be sure to read through those - no chains, I think no restaurants or bars/clubs, etc) But each review is relatively painless. They need to be at least 50 words, which when all it said and done is only a few sentences and only takes a few minutes each.
If you're stumped as to what to review, I looked up my local Chamber of Commerce website and used the lists of shops, services, and attractions. They also had their own descriptions which helped a LOT when I was stumped as to what to say.
It can be a little daunting to think you need to sit and write 30 reviews, so break it up. Maybe do 5 reviews each time you log on. Or do 10 a night. Something manageable. Or if you have the time to sit and spend at the computer, go ahead and do it all at once!
Don't email them that you've finished your reviews until you've finished ALL the reviews you plan to submit. They have a pretty quick turn around for getting the codes for your GCs sent to your email - so you should be able to buy that book within a couple days of finishing reviews.