
Blood and Guts covers the major body systems and has a nice handful of experiments and activities to do, including dissections. The physiology is good, though the simple line drawings don't really capture anatomy well.

Books well written in an engaging style such that they are an enjoyment to read;
Books with high quality information, both in morality and depth.
The industry is clearly more excited about glossy photography and eye-popping illustrations than it is about content and writing. What concerns me most about content is that the middle school literature contains little more depth than the elementary literature, and is thus far behind high school textbooks.
You can search the Massachusetts Curriculum Frameworks (standards) by grade and subject. I compared the grades 6 through 8 standards with those for high school. Is it my imagination or does there seem to be a huge gulf between them? Perhaps I should start eyeing those high school standards more often and searching for books that will bridge that gulf.
A good way to get living books is to be quite specific about a topic. While a search for "ocean" is apt to get you a whole lot of twaddle, looking specifically for "tides" or "currents" will more likely turn up something more in depth and engaging. Don't be afraid of "feeding" your children these advanced concepts. Just like literature, children "hunger" for interesting learning. Don't settle for the Great Illustrated Classics version of science!