I was really amazed by my first encounters with serious mathematics textbooks. I was very interested and impressed by the quality of the reasoning, but it was quite hard to stay alert and focused. After a few experiences of reading a few pages only to discover that I really had no idea what I’d just read, I learned to drink lots of coffee, slow way down, and accept that I needed to read these books at 1/10th or 1/50th standard reading speed, pay attention to every single word and backtrack to look up all the obscure numbers of equations and theorems in order to follow the arguments.
—Bill Thurston, from the foreword to Teichmüller Theory and Applications to Geometry, Topology, and Dynamics
He goes on to talk about the importance of using one’s whole mind in understanding mathematics. You might want to check it out.