Which Apologia books do you have? I might be willing to take them off your hands. We own Astronomy and Zoology 3 and my kids LOVE them. Email me at tdrowlee AT yahoo DoT com if you want to chat about it.
Okay, as for other ideas for science: We loved using the Magic School Bus chapter books one year for unit studies, and there are quite a few series on different levels for Magic School Bus now, I reviewed them on Curriculum Choice here:
http://thecurriculumchoice.com/2010/01/magic-school-bus-books-review/
We also enjoyed reading Crinkleroot’s Nature Almanac:
http://www.amazon.com/Crinkleroots-Nature-Almanac-Crinkleroot-Arnosky/dp/0689805349
There are a lot of Crinkleroot books covering trees, mammals, animal tracks, birds, butterflies and moths, and more. They are sweet illustrations with good information.
Hope that helps some!