I started adding layers in my sketchbooks a few years ago when I realized two things:
One is that I like my sketchbooks to be an honest reflection of my life (which means ordinary repetitive, unavoidably messy, and often embarrassing, filled with all sorts of fleeting thoughts, things, and styles). And two - that I like my books to be chronological.
Before that, I siloed different parts of my life and kept several sketchbooks going - drawings from life, sketches for illustrations, plans, projects, designs, layouts, etc. I still do that sometimes if the project is too big, too private, proprietary or if I have a need to compartmentalize. But because life is impossibly interconnected everything ends up reflected in my main sketchbooks in one form or another - even if it is just a tiny note on the side of a sketch about something completely different, reflection on materials that I am using for a project or thumbnails that I hashed out while reading or learning about something.
The resolution to honor both of my requirements gave me permission and the obligation to come up with systems to make it happen. It took some time and is still evolving but now I feel like everything has a home. I have the freedom to use whatever paper I happened to crave for just this one drawing or to stick tiny notes, doodles, and sketches drawn on napkins, envelopes, receipts, etc., and if I forgot to add a sketch and found it a week later - now I know where to put it so that I can find it again: just flip back to the day it was made and make it work somehow.
Here are three sketches: from Sketchbooks 55, 88 and 99 (from 2011 to 2016) - I started with just sticking a corner of an envelope on a page with sketches while traveling, then began adding some sketches as flaps and the rightmost page from 2016 has some custom cut sketches glued in and a bunch of things on a sketchbook spread and added a two sided flap. And here is a video with a tiny flip-through some of the pages from the most recent book - #138.