Hi,
I had hoped to go further... but haven't yet.
So here is what I've got so far on SDL2 basics.
Apart from reading the source, it's useful to run it.
But I guess you probably know these parts already.
The best resources for SDL2 are...
All function and structure documentation is in here.
Also installation instructions, and how to download the latest code from their hg. repo
These are in C++ but cover many things in a tutorial style. With example code.
Another C++ tutorial series using the medium of Youtube.
This time making a roguelike, rather than topic specific..
These have mailing list archives, which is now a forum.
A good place to ask questions and to see what is happening.
Apart from following the hg repo, here are where developments happen.
Where patches and bug reports land.
Some interesting discussions on things are in here.
Search for closed bugs.
Inside the source, see the header files (which also have some docs) and the test folder.
The tests are actually short example programs, but are also used for manually testing things.
cd test
Finally, because there are very few resources, searching github All repos->Code-> C and C++ files can be useful.