Having been to a number of libraries and used a number of their
   electronic card catalogues.... there's a HUGE variety... and
   they're often very confusing to navigate.

   Now I'm lucky here. Where I am, they all use the Dewey Decimal
   System.

   I also happen to have it more or less memorized from years of
   experience. I can just go in a library and find a book that's
   more-or-less on the subject I'm curious about. I get a 'feeling'
   that it's somewhere between 350-400 for example, and zoom into
   it.

   Could I tell you right now what's in the 350-400? No. But when I
   had a topic in mind, that's when I get the feel for what area
   it'll be in. We have a number of libraries in town with
   different books and different configurations and it works.

   I suspect if I was in a library that used another cataloging
   system, I'd be lost for bit until I understand their schema.
   Thankfully they usually have it posted somewhere.