How to Navigate a Spacecraft to Mars
EccentricAnomaly writes "JPL has an article on how to design trajectories to Mars with the porkchop plot. They use an interesting analogy between reading these plots and divining the future from animal entrails. JPL also has the most excellent Basics of Spaceflight, which is an easy to understand tutorial on how to explore deep space."
You can really get the feel for what's involved in plotting trajectories and getting to the right orbit. It's really good.
And, it's free! (beer)