Demonstrating genuine originality, the publishers of the DK Eyewitness books have put a refreshing twist on the old cliche that "a picture is worth a thousand words." Each book has plenty of pictures, mostly of the photograph kind; but paired with every image is explanatory text that tells you exactly what it is you're looking at.
Sometimes the sheer number of pictures can be a bit overwhelming, but there are plenty of things you can't really grasp just reading about them in a book. Such as the uniform and equipment of a U. S. Marine in the Vietnam War, for instance, or the rigging of a sailing ship. DK Eyewitness books are about the best available for learning of this kind.
The editors take a living-history approach, meaning that they don't just draw pictures, they present actual photographs of people dressed up in period costume. As much as possible, they also provide images of actual objects, whether they're photos taken from museum collections, pictures of old maps, or nature photographs.
Younger kids will likely be enthralled by these, but somewhat older students (upper elementary and middle school) are probably the best audience, since they'll be able to appreciate and learn from the educational text. Even high schoolers can use these for research, or just to get a cursory understanding of a topic before moving on to deeper study.
(DK Eyewitness books are also great resources for IEW students: because the text is short and to-the-point, kids can easily rewrite sentences and paragraphs in the note-taking/outlining phase of composition instruction.)
Be aware that some of the volumes contain a decidedly Darwinian worldview. At the same time, since these are designed for children, you don't have to worry about inappropriate content or crudeness. These are refreshingly straightfoward books, and are sure to provide hours of productive enjoyment for even the most reluctant students.