The two most important resources when developing with JUCE are the official JUCE documentation & the JUCE forum.
The API documentation for JUCE is very good in my opinion. All of the classes & functions are clearly documented.
The same goes for the forum, every time I asked a question, it was answered on the same day, which compared to other communities is simply awesome.
If you like to learn using videos, you should be able to go from beginner to advanced using the resources found mainly on YouTube.
The JUCE team has their own YouTube channel, where yo can find all of the talks from the yearly Audio Developer Confrence (ADC)
. The content goes from general audio development to JUCE basics to advanced, companies showing of their workflow using JUCE and much more. I will link to specific talks in later sections.