Oh, my! I LOVE Library thing! I only just started, but here's the deal: you input all of your books, add your own tags (toddlers, mama-aya, nature, borrowed, loaned....) and comments (loaned to so and so, chapter 3 great for plant cycle lesson....) and you have the option to rate books, give away books you don't want, write reviews and view other reviews, compare your library with others, all kinds of fun, I mean, organizational tools.
I've been trying for a long time to figure out how to organize all our books- not nearly enough shelf space to do it neatly and categorically, and, really, lots of books are good for more than one category, right? So I'm just starting out, but this will work, I think. I'm tagging books with subject and age-group tags, so, for instance, when I'm looking for books to fill out a lesson on animals in winter I'll be able to search my library by preschool and elementary groups as well as season (winter) and subject (animals). The math books I just loaned, well, I'll know who to look for when I want them back, and the stack of science books I've borrowed, well, I'll know who to avoid until I'm finished with them! Just kidding... if you want them back now I'm happy to return them.