Evernote does not have nested notebooks. Stacks (the Evernote term) can *group* notebooks. But stacks cannot contain notes or other stacks.
Sorry, I may have not understood PrecariousPhoto's request properly (I'm french).
"Nested notebooks" and "stacks" looked pretty much the same to me. It's a new level to organize your work by grouping your notebooks (as you group your notes in notebooks).
But now, reading your post BurgersNFries, I get the difference