I've been using the same system for so many years and collecting things for so long, that I pretty much know where every file is.

  Technology
    Programming
      Meta
        (stuff about architecture, design patterns, etc)
      Python
      C
      JavaScript
      Scala
      Erlang
      Lisp
    Agriculture
    Automotive
    Building
      Architecture (books on colonial architecture, way to build, etc)
    Electronics
    Instrumentation & Control
      optimal control stuff, college stuff,
    Mechanical & Machines
    Signal Processing
    GSM
    Systems Engineering
    Design
    Food Tech
    Fluid Mechanics
    Aircraft & Aeronautics
  Core Knowledge
    MIR (books from "MIR Publishers")
    Mathematics
    Physics
  Documents
    Business
      Silicon Valley
        (books specific to sv: High output management, etc)
      Author1
        books by Author1
      etc
    Ideation
    History - Biography - Journals
    Democracy
    Language Learning
    Military
      Weapons
    Intelligence
      (intelligence analysis, etc)
    Cognitive Performance
      (cognition, meta-cognition, "How Learning Works", etc)

  Code lives here:

  workspace:
    musing (snippets I write, refactoring pieces of code, idioms, etc)
    python
      flask (flask repo)
      library_repo_etc
    scala
    c
    etc..

So, if you ask me about Flask's documentation, it's in Technology/Programming/Python and so on and so forth..