Maybe it's not a specific programming question, that's why I got "The question you're asking appears subjective and is likely to be closed." warning bar once I typed the question title (how stackoverflow achieve this? interesting. by keyword?)
Anyway, my question is about Amazon API, because I didn't have previous experiences on that. For example: now I want to buy books on javascript, so that I typed 'javascript' on Amazon under book department. Of course, I only want the best book (the definition of best is to be determined. For example: 0.3*numberOfComments+0.5*rating etc...). Of course, I wish Amazon already provide some API to achieve this instead of crawling the data on its web by myself.