It is very difficult to access except for professional developers and majors. 
If a professional developer can't understand bitcoin, then i would doubt about his professionalism, i mean, I'm a developer but I'm not a professional and i understand bitcoin, it takes me some time and the code level still not clear at all for me, but it's clear to me how it works.
There are books like "Mastering Bitcoin" if any developer reads it it will be easy to understand how it works. Don't try to understand the code, just follow the logic.
One good thing about this forum that I've come to realize is that the discussions here will give you more insights about Bitcoin just like you suggested a book titled "Mastering Bitcoin". I've heard about this book on many occasions but haven't read about it however, I think I'll definitely go for it after today so I can also learn from it.