Just one at a time. I've tried to read multiple books at once, but it never really works for me, I usually just end up focusing on one at some point, and then losing interest in the others.

I get into a flow state of reading, where I'm reading the words with my internal monologue, and I see visions of what's happening in the book, with their clarity depending on how well-described they are. My advice: don't think too much how you're "supposed to" read, just go with the flow.