It's ONLY Rocket Science by Lucy Rogers (I bought the Kindle version). Dr. Rogers is a British mechanical engineer and freelance journalist on space and astronomy topics (perhaps among others).She says on her website that she aims to write about science in plain English, and she pretty much succeeds in this book. For example, her explanations of orbital elements, Lagrange points, and ground tracks are quite clear and include some simple illustrations (including ones for Molniya and geosynchronous orbits). On the other hand, her explanation of rendezvous and docking is rather too brief, and could use some more illustrations for someone who wants the background needed to actually do rendezvous and docking in a real spacecraft (or in Orbiter, for most of us). The book also covers the space environment, rockets and spacecraft, types of missions, propulsion systems, communications, humans in space, and other topics in addition to the basics of orbital mechanics. There's a website with information and book excerpts here.
To Rise From Earth (Second Edition), which is sadly out of print (used copies are available on Amazon). I wrote about this book in some detail in 2006. The subtitle "an easy-to-understand guide to spaceflight" is really true, and Lee (who works as an interplanetary space flight engineer at JPL) covers a lot of ground in this book, including a lot of space history with detailed information on the Apollo program and the space shuttle. In the orbital mechanics area, the explanations are detailed, and there are many excellent color illustrations that support the explanations. Rendezvous and docking and ground track explanations are especially well done. As I wrote in 2006:
Both the book and Orbiter provide an essentially non-mathematical introduction to space flight - the book approaches this through geometry, using a large number of excellent diagrams as well as clear analogies and other text descriptions and many photographs. Orbiter takes the first-person approach of putting you in the pilot's seat and providing various instruments to guide you as you control the spacecraft to change orbits, rendezvous with another spacecraft, or fly to the Moon or to Mars.So if you are a budding astronaut (or Orbinaut), which book should you choose? Ideally you should read both - Rogers covers some topics better than Lee (for example, Lagrange points) and you may find some explanations to be clearer in spite of the small number of illustrations. But if you want to really understand how orbits work and how you change them to get where you want to get in space, you should grab a copy of Wayne Lee's book before the used copies run out. Rogers' 2008 book is still in print (paper and Kindle).