Mark Kurlansky has taken the best parts of his book for adults - Salt: A World History - and made them into an excellent children's book. The history and science surrounding the only rock eaten by human beings is made even more appealing through S.D. Schindler's amusing illustrations.
I learned all kinds of cool facts:"A healthy adult's body contains about 250 grams of salt, which would fill three large saltshakers."
"A wide road near Lake Erie was made by buffalo, and the salt lick found at the end of it became the city of Buffalo, New York."
"In the 1600s, conditions in French prisons were so bad that many prisoners died before they could be brought to trial. So they were preserved in salt and kept until their court date!"
This last bit ties into the adult novel that I'm currently reading, Parrot and Olivier in America (Peter Carey), since the study of prisons in America is the reason Olivier has been sent there by the French government. During French revolutionary times, pigeons owned by aristocrats (such as Olivier's family) went to trial, were found guilty, and then killed for the crime of stealing grain. I'm seriously digressing here.
The Story of Salt will fascinate curious readers in Grade 3 to 7.