Based on feedback from a ton of developers, we've just completed a major overhaul of the Google Checkout Developer's Guide. The new guide is designed to give you an overview of how Google Checkout works and what features it offers. The overview then links to additional documents that let you drill down into the inner workings of specific features. So whether you're selling digital goods, calculating your own shipping costs, or defining obscure tax rules, the new Checkout guide has a little something for everybody.
Stay tuned for updates to the HTML API Developer's Guide.