Buy and learn to build with the WordPress Genesis Framework. When I started using the Genesis Framework my work improved tremendously, not only because it is a great framework but because it showed me the power of and got me using hooks and filters, which are the key to building in WordPress. I haven’t used their pre-built Genesis Child Themes because the of the work I do but I’ve heard good things about them if you are looking for an existing theme to modify. In my opinion, the Genesis Framework is the best $59.95 a WordPress developer can spend. If you are building a Genesis Child Theme from scratch, I recommend my free Bootstrap based Genesis Child Theme.
WordPress Development Books
Professional WordPress Plugin Development – I’ve personally read and can vouch for. It is excellent.
Professional WordPress: Design and Development – I’ve not personally read yet but I’ve heard only good things and look forward to reading it.
Follow WordPress Developers on Twitter
There are lots of great WordPress developers on Twitter. I suggest following them. In addition to links to great articles it gives you a feel for the WordPress community as a whole. Rather than list them here, feel free to check out who I follow https://twitter.com/salcode/following. I’d estimate over 90% of the people I am following are specifically because of WordPress.
WordPress Online Resources
Looking up function names on codex.wordpress.org is an invaluable way to get the proper documentation.
Of course, I’m going to recommend you read what I write about WordPress
WordPress Opportunities in Person
I recommend attending WordPress Meetups and WordCamps. If you are local to me in the Reading area, I suggest the Philly Burbs WordPress Meetup (we can carpool). Also upcoming at the time I’m writing this is WordCamp Philly 2014 and just past is WordCamp Lancaster 2014.