Here is a short list of some resources I’ve found useful for child theme development and WordPress generally.
e-Books
The above three books are absolutely excellent. They are all ebooks with loads of links to resources and articles. They are also updated for each new WordPress version. Get them at: https://wp-tao.com/store/
Web
WordPress Development for Beginners
Learning PHP for WordPress Development
PHP Fundamentals for WordPress
Beginners Guide to PHP for WordPress
WordPress Codex Child Themes entry (check out linked resources at the end of the page)
Some WordPress Action Load Order resources
https://codex.wordpress.org/Plugin_API/Action_Reference#Actions_Run_During_a_Typical_Request
