I just found the Christian Liberty Press Nature Readers at our homeschool library and my daughter who loves science and nature absolutely loves these books! I believe they are part of their reading program so they have Nature Reader K, 1st, 2nd, 3rd, 4th and 5th (I think). She just finished 2nd and is beginning 3rd. The only problem is that I wish there were more of them! :)
Here is the complete set -http://www.christianlibertypress.com/proddetail.asp?prod=KITNR
Melissa