Wanting to know more!

I've started playing around with the firmware a little.
I'm finding that I am a little more interested than I thought I would be.
I'm specifically interested in the Marlin firmware as opposed to firmware in genera.
So I'm looking for something I can read that explains the flow of the firmware.
I'm finding different types of files ie. *.h, *.o, *.ccp etc. I would like to know what they
all mean. What type of information is normally in each file type and which comes first when compiling.
It looks to me like *.ccp come first - but do they?

So where can I go to read about all this without being overwhelmed right away?
A beginners tutorial?

thanks
don