Not all the good nature books out there are really old. On our last library trip I picked up three books in the Young Naturalist series by Gilles Brillon. These were originally written in French (published in Quebec) and are translated by Christina Richards.
I can't comment much on the third one, Discovering the Heavens--but I am very impressed with the other two: Discovering Spiders, Snails, and Other Creepy Crawlies; and Discovering Insects: Ants, Flies, Crickets... There's enough in each of these to keep elementary-age naturalists busy for quite a long time--if they can find samples of the required critters to do the experiments with. (Sample activity: snail race.) There's also quite a bit of vocabulary worked in fairly painlessly. I like the fact (although kids might not care) that the critters in question are divided up by groups: under Annelids, we get to study earthworms; under Molluscs, we look at slugs and snails; under Arthropods, we get Arachnids (spiders), Myriapods (centipedes and millipedes), and Crustaceans (sowbugs).
These look like a great resource, and they're also a good reminder that subjects for nature study can be as close as your own backyard.