The Ascent of Science, and a JPL work called "The Basics of Space Flight."
I wrote about JPL's "Basics" back in 2007. It's a really great educational web site developed by JPL spaceflight engineer Dave Doody. The web site has a number of interactive features (including quizzes) and is maintained and updated by JPL. There have been PDF versions available too, but they have carried a disclaimer that they are not maintained, and past versions were not very nicely formatted (essentially PDF captures of the web pages). When I found a PDF dated 2001, I decided to check JPL for a newer version. Sure enough, I found a 2011 version that Doody had reformatted specifically for e-book and print purposes. It's really great.
The home page for the JPL Basics of Space Flight website is here, and this is definitely the most complete and up-to-date version if you wish to explore interactively. If you want a nicely formatted, albeit slightly older and non-interactive PDF version, click here (it's about 17 megabytes). There is also a paperback version available from Amazon.
In reading the introduction to the PDF, I learned about a more technical book that Dave Doody published in 2009, Deep Space Craft: An Overview of Interplanetary Flight. I read a few pages through Amazon's "Look Inside" feature, and it's definitely a piece of space-geek heaven. But it's a $92 textbook and I wasn't going to buy it (no Kindle version, and I very rarely buy paper books these days). But just now I saw a "like new" copy from a third party seller for $32 so I decided to go for it. What can I say about an addiction?