Just before I released this plug-in Amazon changed their terms so that they only allow a limited number of queries per hour to their database. Yes, even the links on Bookspree.com run out at the end of most hours. The only way to get Amazon to give you more queries is to sell more items.
The amazon error was the best I can do in the latest release. I am working on some better results for a future release but considering the limits, there is not much I can do.
Read below Amazon's Efficiency Guidelines - http://docs.amazonwebservices.com/AWSECommerceService/latest/DG/TroubleshootingApplications.html?r=3305
If your application is trying to submit requests that exceed the maximum hourly request limit for your account, you may receive error messages from Product Advertising API. The hourly request limit for each account is calculated based on revenue performance. Each account used to access the Product Advertising API is allowed an initial usage limit of 2,000 requests per hour. Each account will receive an additional 500 requests per hour (up to a maximum of 25,000 requests per hour) for every $1 of shipped item revenue driven per hour in a trailing 30-day period. You could verify that your sales are being attributed to your calls to the Product Advertising API by checking for the following:
You are using the only links provided by the API when linking back to Amazon.
Your Associate Account and Product Advertising API account are created using the same Amazon Account (or email).
You are passing your Associate tag in all your requests to the API.