Developer Handbook
From Shopify Wiki
Our community’s own Dave Lazar (Resistor Software), aka HunkyBill, has written a book documenting his 6 year career as a developer on the Shopify platform. Dave talks about what it’s like to work with customers, his workflow, and answers to questions his younger self and other developers new to the platform asked.
Want to suggest some changes or new material? GitHub pull requests are more than welcome. Find the book’s open source/Creative Commons licensed repository here: http://github.com/shopify/Shopify-Developer-Book