Decent general overview article
Shopside Blog Series
Six part blog series covering the development of their first app
Also by them, this could come in useful:
Shopify App install notifications to Slack without code
Autocode App for sending Shopify Partner API notifications to Slack.
Shopify Nation blog posts
Code Shopify blog posts
First app in the Shopify App Store
Guide To Shopify Webhooks Features And Best Practices
80+ tips around Shopify Apps
4 Questions To Help You With Shopify App Due Diligence
Track your Shopify app listing using Google Analytics in the new App Store — Dec 2022 onwards