This questions has not a single answer and you should to explore diffrent opinions in order to find the ideal for you.
My opinion is to not read the source code files like a book you will not gain anything, Back in school, when student reads something he thinks that understand it..But, doing is where the real learning takes place, you can create an altcoin based on bitcoin 0.9v its easy you have to change only 2-3 lines of code, and start perfome changes to code then test test test!!