It depends on the type of book.
If it’s something that’s really dense and packed full of ideas, I’ll read a little snippet of it. The Imitation of Christ is one book like this. It’s already chopped up into nice, bite-sized chunks, so that’s really helpful.
If a book has a good flow to it, I’ll read as many chapters in a session as I can. Like my Sayers or my Christie or whatever.
Some books have different paces at different points. Agreda’s City of God has points where I can read fifteen chapters in a sitting and not get tired; or it has points where I’m slogging through one chapter and the words are just kind of piling up and not really hitting any emotional chords and “when are we going to get past this part?”. The Interior Castle is one where one chapter can make 100% perfect sense, and another chapter is so far beyond anything I can relate to, it’s too abstract to properly appreciate.
I generally try to have one book that I’m reading at a time, for fun. And I generally have one book that I’m reading at a time, to better myself in some way. What ends up happening is that I usually end up re-reading the ones I’m familiar with, or just kind of randomly grabbing a chapter and then sticking it back on the shelf. I need to be more methodical in my nonfiction reading.
For fun, I do like reading short stories more than I like reading long novels. My attention span was better when I didn’t have kids or responsibilities to compete with my focus. But even so, you can get so much in the zone that you can read five or six pages and have no clue what you just got finished reading.