Anything Roald Dahl is awesome. His chapters are really short, too, so you can read a ton if the kid gets into it, but if you have to break off and cut it short one night, there are easy places to pause.
I've noticed a cognitive leap between five and six, too--last year I picked out books to read aloud and my son couldn't quite hold a story that big in his head at once. But this year, we've revisited some, and he's really loved them. (The True Meaning of Smekday, being one such book. Also, be warned, it has no chapter breaks!) For a 5YO, I do like the idea of books where it's not essential to hold every single plot point in their heads to understand the story--the Little House books, for example, are more slice of life, and a little easier to take up and set down over time. (Plus of course, they are interesting and well written!) Some of the shorter/younger Newberies might work, too (Charlotte's Web or other books by the same author might be good.)