There are tons and tons of wiki and blog posts about bitcoin out there, but in my opinion the original paper is really good, if you have a technical background. I don't mean like crypto-expert, I think it's something the average programmer can read, maybe with a bit of googling. I usually wouldn't recommend a paper, but I found it really clear and straightforward. Maybe it's because I already understood bitcoin pretty well, but here it is if you want to take a look: http://bitcoin.org/bitcoin.pdf

The bitcoin wiki is also pretty good: https://en.bitcoin.it/wiki/Introduction