I've been struggling with the Amazon Associates Web Service for some time now, using the simple "3.0" version with some online resyndication tools to create some pretty useful dynamic content. (At least I think it's useful!)
When I learned that the deprecated 3.0 version would expire this March (March 1? March 31? I don't know.) I broke down and ordered Jason Levitt's The Web Developer's Guide to Amazon e-Commerce Service, but didn't get serious about "Migrating from Amazon ECS 3.0" until quite recently. Big mistake!
While Levitt's book is extremely useful, it is a bit out of date, which wouldn't really matter much if Amazon's support for older ECS 4.0 versions was better. Unfortunately, there are a lot of holes in Amazon's documentation of the many sub-versions (or "subversions") of ECS 4.0 denoted by dates in YYYY-MM-DD format.
As if to make a bad situation worse, Amazon has timed the rollout of it's newly renamed Amazon Associate's Web Service to coincide with the expiration of AWS (E-Commerce) Service 3.0 -- Yikes!
I've yet to see any indication that A2W (as they like to call it) is anything more than an incremental 4.0 upgrade, but it does serve to further muddy the documentation waters.
There really seems to be no alternative to really learning XML and XSLT fast. Here are a few of the books I'm using in this quest: