OK, so this isn't one for your average 9 year-old, but for a teen (or older) who is REALLY into electronics, this is one amazing book.  What makes it more incredible is that it's been the Electronics Bible for several decades already.  Tom (Tech Camp's MD) stumbled upon this as a teenager himself (definitely several decades ago), and it's been the go-to reference ever since.

It's an unusual combination of somewhat-academic theory but with a huge amount of practical experience thrown in, and explanations that are down-to-earth enough to get a good understanding of electronics from first principles.

It's now up it's third edition, and there are some add-on books published now as well.  If you're in the UK you can get this from Amazon.  On the US-side, we suggest you get it from Adafruit, which is one of our favourite online stores.  Along with Horowitz and Hill, the book's legendary authors, Limor Fried who is 'Lady Ada' and the founder of Adafruit Industries is one of the leading lights of open-source electronic design this century and of women in STEM.