The Manga Guide to Relativity
Physics is an intimidating subject this (comic-)book manages to tackle in a way most teenagers will find easy to understand. More to the point, they will, hopefully, find it fun and compelling too. Its structure is a good fit for high-school teenagers (don't traumatize your 10-year-old kid with it) with an interest for science and/or Japanese comic-books.
The end-of-chapter lessons are a welcome add-on for the kids really interested in the deeper aspects and capable to understand the mathematical notation used. Like I said, physics can be intimidating and most kids aren't really introduced to relativity until well after they can't be called kids anymore (sorry - science fiction, despite its noble efforts, won't teach them).
As a father, I found the book both entertaining and useful as a source of easy ways to explain this subject to my own teenager. He is next in line to be subjected to the book.
The PDF version worked well on my 1st-gen Nook and the text in the graphical pages was readable without zooming or rotation, if a bit small. The end-of-chapter sections needed zooming in.
Relevant disclosure: I reviewed the PDF under the O'Reilly reviewer program.