This is an idea I've been having since a long time. I think it is relatively easy to implement as well.
We need textbooks like we have online maps. Textbooks that give you an overview first and then let you zoom in to any part and get more and more details. The deeper we go and the more details we have the harder will the level of complexity be. So, a beginner can probably zoom out and get a large overview of all the topics they need. Someone who already has the overview can zoom in at a part and get some more details. Then, they can zoom in again and get more details, and again, and again till they reach the maximum available information.
Writing such a textbook may seem complicated but all it takes is some amount of reorganization of thoughts and marking sentences by their level of complexity.