Fandex Family Field Guides: Workman Publishing
The Fandex series has changed our lives three times. Workman publishes these book-mark shaped reference books on at least fifteen topics, including cats, trees, first ladies, and butterflies. The books are bound at the bottom by a pin, so the pages of the book fan out - hence the name Fandex. Each page covers one instance of the book's topic - for example, each page of the Trees Fandex covers one species. The top of each page is a cut-to-shape painting of the page's subject.
So far the three books owned by my older son have each, in turn, led to multi-month obsessions involving the whole family, starting with the 50 States Fandex, followed by Presidents and Mythology. The only failure we've had was the Civil War Fandex - maybe we're not quite ready for that one, or maybe the subject doesn't fit the Fandex format as well.
I don't know what it is exac tly, but kids seem to love these books - the reviews on Amazon tell me my son isn't the only one. My son says it's easy to find information, because the shape and clever fan-out design makes it easy to flip to any page almost instantly, and the picture tells you when you've arrived. He also says they're easy to hold and carry - he should know, since he has taken his Fandexes everywhere. He likes that there are levels of information - each page has a picture, some quick facts, and then more involved text below. The pictures that top each page, far from being afterthoughts, are full-color beauties.
The really great thing about these books, at least in our family, is that they are such great starting points. They provide a familiarity and working vocabulary that makes more complex references suddenly accessible.