[Adoby]

You can also just add a custom column to the calibre library. Then put the classification there. Then, when you save books to disc, you can use the classification to create subfolders accordingly. In however many levels you desire. The only problem is that each book then only can have one single classification.

If you instead access the books from inside calibre you can use the search functions and possibly also virtual libraries and have books belong to any number of classifications.

So for instance a book with tags algorithms, computers and data structures can be simultaneously classified as part of:

Books on computers
Books on algoritms
Books on data structures.
Books on computers, algorithms and data structures
Books on algorithms and data structures
Books on data structures and algorithms

And so on. Whatever you find meaningful and want to search for.

I assume that by "leaf" you mean "book".