In the meanwhile though, I was able to finally finish reading Game Engine Architecture by Richard Lemarchand and I got to say I'm very satisfied. I think its the best jumping-off point for further learning on the theory and software engineering practices behind a modern Game Engine System. This book explores the various subsystems and the software interfaces that comprise a game engine such as the rendering engine, the collision system, the physics simulation, character animation and audio engine. And it does this at great length without becoming boring. Furthermore, there are code snippets throughout the book to illustrate the various algorithms and data structures discussed. I found these to be very helpful for better grasping the concepts. Last but not least, the author provides software engineering insights, best practices and tips at every corner. Which is great because that's one if the reasons I've decided to embark in this game engine exploration adventure: to learn more about the best software engineering practices out there. Did I mention that the author has earned his stripes in the game development community? He was involved in the development of Naughty's Dog The last of us... So inspiring! This makes his insight and suggestions even more valuable (to my eyes). I can only dream of the day I become part of one of those elite game development studios out there.
In conclusion, I highly recommend this book to anybody approaching game development from a software engineering perspective. With the knowledge gained from this book I feel more empowered now to keep learning more about the Unreal Engine and eventually start messing with its source.