I wondered what cosplay was too. Now you've explained it it all makes perfect sense. Have you ever noticed how often that happens, once someone has explained it to you you wonder how come you never got it before?
I don't think it's nerdy at all. I think it sounds like a lot of fun, and I wish ANYONE I knew did ANYTHING even halfway as interesting as that.
I admire anyone who is so dedicated a fan of anything. To go to so much trouble to get just the right look, sometimes even going so far as to purchase items from the actual shows/films in order to look as authentic as possible.
I know exactly what you mean when you say it is like the books have already been written.
With mine it's like it's a life that has already been lived, and I am simply following the thread of that life picking up the story as I go along. It is almost as if instead of being the writer I am the first person to read the story.
Not that I am an author, or even that experienced actually. I am just writing a book, but I am having a blast.

With cosplay as a part of your story that must open up a ton of new plot adventures and possible endings. It sounds like a great idea, that I wish I'd thought of.
They sound like they would be almost as much fun to write as I am sure they are going to be to read.

Hey maybe if we finish somewhere around the same time we should swap and see what the other's book is like? Of course I promise not to say a word to anyone about what your book is about. Obviously my lips would be well and truly sealed!