AIM and SOAP are historical APIs. The best choice is MWS, which supports all features of AIM and SOAP (i.e. all listing activity, inventory reports, order reports, order acknowledgment and cancellations, refunds, settlement reports). MWS is also well documented at mws.amazon.com, example client libraries are provided for Java, C#, and PHP. For feed and report formats, you should refer to Seller Central Help pages for all formats (unless you are a Marketplace seller, in which case the help pages are accessible from your seller account page).
You may also want to peruse www.amazonservices.com
Finally, here's a link to the SOA-GuideToXML.pdf document, which describes the details of the XML feeds and reports, as well as the overall workflow: