I also find reading multiple books in parallel really helpful. In this way, you can read multiple kinds of books.

For example, I am currently reading a "mathematical book"- Jordan Ellenberg's 'How Not to Be Wrong', a big fat novel in my native language, and Rumi's poetry in English.

It get's tiring to read serious non-fiction for multiple hours but there is still appetite for reading left. You should switch to a fiction then. And read poetry and book like Meditations by Aurelias now and then, maybe not even linearly.

I have found this combination to work for me really well.