Books where dreams are more than just dreams

I’m looking for a book where the big theme is the protagonist’s dreams, but they’re more than just dreams – whether they’re a window to an alternate life or an alternate reality. Or perhaps the dreams are actually the “real world” and the protagonist’s waking life is the illusion. Bonus if this is tied to the protagonist’s mental health somehow, but not necessarily in a cheap “he was crazy the whole time” kind of way. Much appreciated!