MMORPG (MMO) Textbook
There is learning about an MMORPG because you are a player and a fan and awesome enthusiast! And there is learning about an MMO because you would like to create one as a game developer. This is meant for the game developer more than the fan, more for people who want to work in an AAA company and have no idea how an MMO works, what it is, or why they are so popular these days. If you would like a non-judgmental space to learn, here is suitable for you.
The point is not to know everything, but to expose yourself to important concepts and structures that are common to all MMO companies. Remember that every game is different, but at the core, the components are the same. Different skins, same bones. Read diligently and study well.