Toadstool Bookshop in Keene, NH last weekend (great bookstore, BTW). Voyager's Grand Tour: To the Outer Planets and Beyond by Henry Dethloff and Ronald Schorn (2009). Among other things, the authors do a great job of setting the scene in terms of how little anyone knew about the outer solar system before Voyager 1 and Voyager 2 flew by Jupiter and Saturn between 1979 and 1981 (and in the case of Voyager 2, Uranus and Neptune from 1986 to 1989). Pioneers 10 and 11 had gathered some information (we were really clueless before 1973), but the quality and quantity of imagery and scientific data was so much greater for the Voyagers.
The moons of the outer planets were hardly known at all, but were somehow considered unlikely to be very interesting. Voyager 1 dramatically altered that view when it sent back imagery and data showing that there are active volcanoes on Io. The Voyager program was an amazing success. Of course we now know much more about Jupiter (Galileo orbiter) and Saturn (Cassini orbiter), but Voyager 2 is still the only spacecraft to have visited Uranus and Neptune.