Yes, textbooks are a good way to read mathematics as long as they aren't too far away from what you currently know and the subject is interesting enough to preserver through the confusing parts. If you don't understand the purpose of what you're reading, I find it best to skim the section and return later to it once you have a clearer idea. Otherwise reading slowly, rereading parts that aren't completely intuitive, and trying to figure out why it's hard to understand is the way to go. Also for terms I do not know I will search them up. Although I tend to read more papers than textbooks, especially PhD thesis. Papers are nice because they're shorter and PhD thesis, while long, tend to cover topics you won't find in books. Also usually I'm reading something to try and do something else so there's some context going in which I think helps with understanding.