General developer forum

There are no up to date books detailed books on Moodle development, though Tim Hunts chapter on the architecture is highly recommend.  The best tool in my view is an IDE such as Netbeans or Eclipse that shows you the ancestors methods of classes. Whenever I try to do something I first find as much existing code that does something similar.

As Marcus said - http://aosabook.org/en/moodle.html

I just wanted to add that the other way to learn more about Moodle development is to do it. Active learning and all that.

And, if you share your Moodle code, you are likely to get feedback. If you get to the point of submitting a bug fix to the core Moodle code, you will get feedback as your code goest through review. If you upload a plugin to the plugins database, to share it with the community, then you will also get some code review. Specific feedback on what you have done is one of the best ways to learn, and if you fix a bug in Moodle, or share a new plugin, the whole community benefits.

You are right that the resources for new Moodle developers are not great. The best three resources are the Moodle developer documentation, these forums, and reading other people's code. 

I have been teaching the MoodleBites developers course (as mentioned elsewhere in this thread) Having to teach this stuff really made it clear to me how much there is to know about Moodle. We are going to be refining that course and maybe bring out some less heavy duty ones.

My advice though is just to get started with something small, and post a lot of questions here on the forums. You won't understand everything you are doing, but you shouldn't worry too much about that. Its a lot like like that when you learn a language like Japanese or Russian too. You can't wait till you know everything completely, before you use it. 

Having contributed to many open source projects in the past, I can initially say that all large projects take time to get your head around, no matter how well documented and how much help you have.  It's the same as starting a new job and just expecting to know everything about the workplace that you need to know.

My experience has been that looking into and resolving bugs is always the best way to get started.  It allows you to have a real world problem to solve, it ensures you have lots of capability to read what's going on and you only need to make small adjustments to the code to get the desired result.  If you attempt to build new functionality up-front, you can get hit with the problem of not knowing the API at all.  This can easily lead to re-implementing existing API's or not using them at all and trying to everything yourself.  Both result in bad outcomes in Moodle.  I've spent some time supporting situations where that has happened.

If you want to be able to learn development, it is critical you have a good IDE with auto-completion filled and function descriptions working so you can quickly and easily find functions you need.  Learning the naming conventions help with finding function and class names.  Debugging setup in your IDE is critical for 2 reasons.  1, you can see what's happening for the bug you are investigating and 2 you can see what all the variables are at different points in the code and what they are doing.  Having been programming on Moodle for at least 5 years I still discover new information about parts of the code by seeing what objects are in the debugger in a certain function.

Aside from actual coding, conducting Peer review of items in the tracker in the 'waiting for peer review' status will allow you to understand what others say about that code as it's reviewed by them and for you to provide feedback to others for things that you know about.  It allows you to see real world code that will go into Moodle, it will let you get skills at using the workflow and tools for bug tracking that Moodle use, including git merges.  You will get experience conducting testing of issues which you will need lots of if you are developing your own code.

Even though there are comments indicating that there isn't great documentation, there is https://docs.moodle.org/30/en/Development:Developer_documentation as a reasonable start.  I have contributed code to projects that make the amount of documentation for Moodle one look like it's the best project in the world.  It's certainly not perfect, but the best way to improve it is to engage with it.

In terms of helping improvements for newcomers, you are in a golden position at this time to be able to do that.  Many of those here have been doing this a long time and it's so second nature now that it's difficult to impossible to work out what the stumbling blocks of a newcomer will be as the answers are just in our brain now.  So improving documentation for newcomers as you learn will provide a significant advantage to yourself and to others.