I tried to read the white paper but it glossed over technical details (for instance, what does "signing" actually entail?). Everything else is either very hand-wavey and high level or assumes you already know how it works.
I'm interesting in understanding the spec and potentially expanding on it or being able to understand the ongoing issues with Bitcoin
In his white paper he only describes the twist, so you won't get a complete overview on how bitcoin works (even if you know how the stuff he's building on works).
This link has been posted to HN before and gives a pretty good overview on how bitcoin works: http://www.michaelnielsen.org/ddi/how-the-bitcoin-protocol-a... So you won't get
A blog post: http://www.righto.com/2014/02/bitcoins-hard-way-using-raw-bi...
A paper: https://eprint.iacr.org/2015/261
A book: https://d28rh4a8wq0iu5.cloudfront.net/bitcointech/readings/p...
Choose your adventure ;)