The first book: #1 The Arabian Nights
Finally, over 140 nights in, the king asked for a story - a breakthrough in any other point. And a nice breather, as the tale of Kana-ma-kana was going on for quite a while. it's probably the longest so far and that made it more difficult to keep track of through the reading sessions I've had.
The nights as a framing device have been interesting as well. Although early on they have some story content, about twenty nights in each night starts with the exact same phrasing, just a different number for the night. It's clear these would just have been the breaks for the storytellers, separating them. Not even always in obvious places - rather than cliffhangers, there are several places where they interrupt the action for no real reason other than length - especially where there are just treatises on asceticism. Even so, as we move between longer stories - and, I believe, different sources of tales - the length differs, and the first 40 or so nights are each noticably longer than the next 100 or so. It's been interesting looking at the stories on that level as well, how the different sources move from the actual tale as at least some vague story to just using it as an almost ritualistic framing device.