@alyssa the best advice I've ever seen is that reading textbooks, as opposed to ordinary books, is not a linear experience. A normal book is read back to front, once, and you get the intended experience from that. But a textbook should NOT be read this way. When I read textbooks, I tend to read the preface/forward and see the general approach the author is taking for the material. When I get to a chapter, I like to read the beginning and the end to see the "shape" of that section before digging into the section as a whole. Then I get into the section.

Repetition is essential. I reread difficult portions of the text to make sure I can still follow the reasoning.

The other advice I have is that the act of reading a textbook is active, not passive. You should be critically responding to what you read, and collect questions for yourself. Have a laptop open next to you so you can chase down rabbit trails of questions you might have. If it's a technical book, a sheet of paper should be next to you at all times so you can rederive the equations the book gives you. It's not uncommon that I spend and hour or more exploring tangential topics to what I'm currently reading. Sometimes, this will lead you to teach yourself something the book planned to teach you later. You will never forget something you teach yourself in this way.