Head on over to DrupalCommerce.org. On the site you will find a nice overview of the original 1.x build and links to all the relevant user guides. Linked below for your convenience as well.
- User Guide - A site builder guide with lots of screenshots that covers installation, order management, product creation, and a myriad of other topics.
- Developer Guide - A guide built for developers that outlines architecture, building payment gateways, code workflow, and utilizing core APIs.
- Commerce Kickstart 2 - A guide built for site builders that are demo’ing with Kickstart 2.x
- Commerce Cookbook - A cookbook of common site builder tasks that covers things like shipping, inventory, reporting, merchandising, etc.
- API Documentation - The doxygen output of all the code documentation that ships with all Commerce installations.