If you already have some programming skills (especially if you are familiar with PHP), and are looking for resources on how to get stuck into WordPress development, then the old idea of sticking your nose in a text book is probably not your best approach.
Text books do quickly go out of date, and are probably better for absolute beginners. Especially in an environment where you are working and creating on the internet. Obviously, your best resource is a the internet itself.
So, here are my top tips for getting started. Yes, it’s a bunch of reading, but it is also a fantastic set of resources that I still reference now: