Reddit mentions of Professional WordPress Plugin Development

Check out Professional Wordpress Plugin Development. One of my favorite books.

What I always suggest is http://www.amazon.com/Professional-WordPress-Plugin-Development-Williams/dp/0470916222

I found writing a theme was more design oriented than any actual development where a plugin by its nature takes more development.

So think of a plugin, big or small, then dive in to trying to make it happen.

Being a senior WP dev to me is understanding WP concepts such as hooks, filters, internal API:s etc.

Read this book from start to finish and you can call yourself a senior WP dev:
http://www.amazon.com/Professional-WordPress-Plugin-Development-Williams/dp/0470916222

Lastly, I've met quite a few senior WP devs who are pretty bad to be honest, so you'd be in good company regardless. ;)

Whenever this is asked I always recommend starting where I did: http://www.amazon.com/Professional-WordPress-Plugin-Development-Williams/dp/0470916222/

here's a book that I used when I was learning and it helped a lot. It's a tad outdated but there's still tons of usefull information.

Professional WordPress Plugin Development is great. A couple of years old, but should still be relevant.

In addition to the Codex I'd recommend reading Digging Into Wordpress - the Book followed by Professional Wordpress Plugin Development. Although the latter is geared towards plugins it goes over in great depth the API. I'd avoid WP Tuts (and similar sites) as a lot of the content in incorrect or deprecated.

I'm awaiting this book, it looks like it's going to be good.

The best that I've found and recommend for learning advanced development with WordPress is called Professional WordPress Plugin Development by Justin Tadlock, Ozh Richard and Brad Williams.

Amazon Link: http://www.amazon.com/Professional-WordPress-Plugin-Development-Williams/dp/0470916222