I was enjoying myself quite well with the game for a few hours. Then I discovered the bookstore, where I looked at probably all the book titles for at least an hour, maybe two, which led me to a post on the development blog. I later spent a significantly longer time browsing the library.
A couple of weeks later, after I’d finished the game, I read or at least skimmed the full development blog, listened to all of the podcasts (not always attentively, but while washing the dishes and stuff like that) and ultimately ended up on this forum.
Basically I was a little disappointed that I hadn’t learned of the game’s existence prior to GOG sending me a promotional e-mail about it.
Anyway, the fan-added value in TWP is my most unique association with TWP. I’m not personally familiar with anything like it. (Not counting the MI2 library.)