A metacognition trick I use is this.
Read the book for 10 minutes (no distractions). Those 10 minutes should include understanding the material I just read.
Am I bored now? Yes = Stop, No = Continue for 10 mins.
I repeat step 2 for a maximum of 30 minutes, then take a break. If I am bored after the break then stop. This method stops me getting distracted and allows me to stop when I know my brain is wandering off.
Reading is one part of it. Also if there are exercises in the material, do them. Lastly if I know I need to read back over the material I highlight the key points (not the whole page!).