Magic School Bus Series by Joanna Cole
Most homeschoolers are familiar with this highly popular series. My boys very much enjoy both the series and the videos, reading and seeing them so many times they know well much of the information from them. The format appeals to even preschool children, yet they cover upper elementary, lower middle school science topics.
We prefer the books written by Joanna Cole and illustrated by Bruce Degen; they are the cream of this crop. The books based on the TV show are good, too, but fall short in writing and illustrating as you would expect from a knock-off. The chapter books have the science content without the wonderful illustrations.
Because the series is so popular, the books are readily available from either the library, used book sales, or any book store. We have just about all of them on our book shelves.