We all REALLY like this book, a library acquisition that I will probably end up buying to add to our permanent "home library."
There are enough visuals - photos, diagrams, illustrations etc. - that my not-quite-reading 5-year-old can enjoy this encyclopedia on her own, but enough text that we can all cozy up on the sofa and learn with it, too. This is a book that will really grow with my kids - offering knowledge on any level from this age on up.
In case you're concerned because of another reviewer's reference to the book's treatment of human sexuality, there is literally one reference to a man's penis "entering" a woman's vagina. There are also detailed diagrams of both organs on that page.
The very youngest kids probably won't be able to read this for themselves, so you can omit the specifics if you prefer; there is no other reference in the book, though on the next page, a developing embryo is shown, along with a pregnant woman.
Frankly, I wouldn't trust a book on anatomy that LEFT OUT this information, and I believe that any child who has been prepared from an early age with a basic understanding of where babies come from will not be shocked by (or even at first, perhaps, very interested in) the specifics.
As a religious person, I'd see it as my obligation to not only make this information available to my kids, but also use moments like this as an opportunity to talk about the sanctity of their bodies and the marriage in which I hope they will create their own babies someday.
The rest of the book seems unlikely to stir controversy - and quite likely to hold kids' interest, especially with its many opportunities for kids to explore their own bodies - listening to their heartbeat, finding the blind spot on their retinas, etc.
Each page offers a "find out more" box in the lower-right corner with "links" to pages offering related information. A page on sickness offers a quick link to a page on infection, for example. I like the way this book mixes serious scientific knowledge with a sense of fun and fascination.
I have been more impressed by the actually written content of this book than I have with similar offerings from Dorling-Kindersley and other educational publishers. I find the DK books are very busy, and suspect they pander to TV-oriented kids with short attention spans.
The text of this book is well-written and complements the illustrations nicely, which means it will make a fine addition to our homeschool living things science next year as part of our classical / Charlotte Mason approach.