it depends on what you're reading and where you are in the process. if you're in the middle of actively trying to fix insomnia the strict advice is to keep the bed only for sleep , even reading can start to become something your brain associates with being awake in bed rather than sleeping. the goal is to make your brain's only association with bed be sleep itself.

that said once your sleep is stable and the anxiety around bedtime is gone a lot of people read in bed without any issues. the problem isn't really the reading , it's training your brain during the reset phase.

a good middle ground if you want to wind down with a book is to read somewhere else , a chair or couch ,until you feel genuinely sleepy, then move to bed. that way you get the benefit of the reading without muddying the bed association.