There's also this recent piece [1], which was lost behind the Medium paywall until republished (apparently just today?) on urbit.org. It's a very solid introduction to the platform, its intent, and the reasoning which underlies it, and it's refreshingly free of jargon. If you're looking for something to whet your curiosity, I can unreservedly recommend it.

[1] https://www.dropbox.com/s/cpy5vt4zfuwgzax/the%20not%20so%20d...