The Daily WordPress Reference is a very handy WordPress email newsletter for developers. I only recently discovered it and subscribed immediately. Each newsletter contains one function, a description of what it does, usage, parameters, return values and a few examples of how to use it. At the end it links to the codex where you can find out more information.
You can view past issues of the newsletter on the Daily WordPress Reference signup page to see if you like the content they’re sending.
Here’s an example of what the newsletters look like in your inbox:
All in all it’s like reading one page of the codex every day. You get bite-sized learning bits conveniently deposited in your inbox, so you can keep expanding your knowledge while on the go. I highly recommend the newsletter to anyone developing for WordPress. The Daily WordPress Reference is totally free so go sign up today!