It could be that I'm the parent of a 7 1/2 year old boy. It could also be that the book has such great footnotes ( when I was in grad school working on a paper for a group project, one of my fellow students commented that I wrote great footnotes), but I found Mary Roach's book Gulp: Adventures on the Alimentary Canal to be very engaging and entertaining, as well as educational.
Gulp is a very approachable "top-to-bottom" tour through the human digestive system. Combining fact, history, some editorial commentary, and much humor you can't help but get pulled into this book, and even pull in those around you. At various points I found myself laughing out loud, leading my wife to comment that I, have a lot more in common with our 7 year old, or at least that there is a certain timelessness to that sort of humor among certain demographics. The chapter on flatulence was particularly amusing, and was a great example of how Roach uses humor to help us learn about socially awkward topics, and to make details memorable.
One of the charms of this, and the other books of her's that I have read, is that she is adept at staying on the correct side of the boundary between humor that makes us comfortable (and makes facts memorable), and humor that is just awkward. She did a similarly good job with Stiff: The Curious Lives of Human Cadavers.
While this isn't the book to read for serious research, it is a good place to learn some fun facts about a subject that is not often discussed in polite company. With footnotes and references, you also have a good starting point in case you want to learn more. The only down side of reading the book is that you may find yourself laughing out loud or at least finding it hard to share a particularly amusing storing with whoever is sitting next to you while you are reading.
Gulp is a great example of how you can be entertained, informed and educated at the same time.
Gulp: Adventures on the Alimentary Canal