One of the favourite (useful) things that each Squirreling did during homeschool was learn something about libraries and the Dewey Decimal System.
("Who's Dewey?" shouts D.W.) [link updated 2011]
We talk about its advantages and shortcomings (who'd think to look for The Three Little Pigs in the 300's? How come our library used to catalogue homeschooling under Domestic Education?). The girls get assignments like "bring home books from three different hundreds sections." They end up with at least an idea of which end of the library to go to if they're looking for history books, or riddles, or books about French braiding.
"Do We" Really Know Dewey is a kid-friendly online guide to the DDS that's been around for years--since the Apprentice was learning these things. The space-alien story still holds up pretty well.
There are some other sites that have come up since then, that I've been using this spring with Crayons. There are some fun ideas here, including the address of these task cards that can be cut apart and used as a Dewey Drill (we're doing a few of these every couple of days as review). "(You need 5 folktales--what section do you look in? You are doing a report about at least different kinds of whales; Your project is to compare lions and tigers; You need to do a report on hurricanes; You need pictures of people in the army" and so on. If you don't like the one about "You are being haunted by ghosts," you can just leave it out.)
As part of our library unit, we've also read The Boy Who Was Raised by Librarians and My Librarian is a Camel. I also took The Library Book and a couple of others out of the library, but decided they had more information than we really needed for grade two.
"Who's Dewey?" Now we know.