I’m a very experienced designer + front-end developer and am now looking to start coding my own WP themes!
I’m curious whats the best resource to begin my journey down that path, considering I have the knowledge stated above and a few months of dedicated time. Are there any top notch ebooks that teach how to install a theme into one of the best frameworks?
What is the most popular framework among the most popular selling themes? (Not counting the ones they made on their own)
You can try some great courses here: https://tutsplus.com/?s=WordPress&post_type=course also some ebooks https://tutsplus.com/?s=WordPress&post_type=ebook also this site http://code.tutsplus.com/categories/wordpress and this one if you have any doubts http://codex.wordpress.org/
If you are not familiar with PHP you can use this course https://tutsplus.com/course/php-fundamentals/ and if you want OOP in your theme learn the basics here https://tutsplus.com/course/object-oriented-design-in-php/
I heard this is a nice framework http://notes.envato.com/industry-news/introducing-the-redux-framework/ also you have see some framework suggestions here in forum, try search.
Those are some really helpful suggestions by Laranz
Additionally you can checkout http://make.wordpress.org/docs/theme-developer-handbook/. Though they are still developing the content here. It’s a good starting point.
To explore wordpress more deeply chekout Professional WordPress: Design and Development book.