>At the risk of sounding quite silly, how do people read these textbooks

literally just front to back. I did maths in university and when I started to work for a few years I didn't get too do much of it so I just got into the habit to put half an hour a day aside to work through whatever books I find interesting.

I actually enjoy it much more now than I did in uni given that I can do it at my own pace now and for fun.