There are innumerable textbooks describing orbital motion and Keplerian gravitational dynamics. One that I find particularly useful is "Orbital Motion" by A.E. Roy, which seems to be out of print, but is probably available in the your local library.
NASA's Jet Propulsion Lab has a very good website called the Basics of Space Flight which actually has a lot of in-depth material. In particular look at sections I.3-I.5 on gravity and orbits.
The Voyager Project Homepage at NASA has a lot of interesting information of the Voyager probes, including their current positions. It contains summaries of the scientific achievements of the Voyagers, as well as dozens of images taken of the outer planets.
Gravity Assist Orbits:
Again, real spacecraft often use Gravity Assist orbits to pick up speed and complete the mission much faster than a classic Hohmann orbit would allow. For more information on Gravity Assist orbits, check out this part of JPL's Basics of Space Flight which describes these orbits in detail (look about halfway down the page...).