I'd start with the bluepaper. It gives a nice high level overview of the Steem blockchain. After that, the whitepaper goes more in depth on technical stuff and implementation details. There is also the Developer Portal, which has a lot of info on all the code related stuff. Regarding the UIs, I don't know of a lot of documentation on that (beyond the dev portal). The GitHub repositories for the individual applications is probably where I would turn, but that would be strongly on the technical side.