I read about GTD and watched some videos when it was released, but never got around to reading the book, may do that.

Nice books! I enjoyed Discrete Maths in college but never really used it anywhere after that.

Well, I believe LibraryThing has been around since before Goodreads, but I can be wrong about it. For cataloging books, it is pretty much the best site around (though I haven't really checked up any other sites in last few years), you have the ability to select the correct edition of your book, have proper cover page etc. There are also library related features if you are interested in that. All accounts can also act as a mini library, giving you complete features of lending etc.

I don't really care about social features much so after trying multiple applications / websites settled on LibraryThing more than a decade ago.