Mark Brownlow wrote an extensive article on open rates. This is how he describes it himself:
A comprehensive guide to everything to do with email open rates, including how to improve them, interpreting rates, measurement issues, and finding averages.
It's written for both newcomers and those with long experience, taking you through from basic issues to advanced considerations in a 4000 word free guide.
And if that's not enough, it includes many links to other articles on the same topic.