According to my book, these sets were very common and large quantities were given away. With the envelope containing a series of 12 cards the value is $7. Individual cards are valued at $1 each. If you have the double sets with two series of cards per envelope, it is valued at $18. The entire set of 96 cards in the box is valued at $60.
Recently, however, I have seen a single series selling for $15. Pricing always depends on how much a collector is willing to pay for an item or series of items.
The whole set consists of the following series...
Series 1: Earth and Air and Sky (Illustrated by Lynn Bogue Hunt)
Series 2: Man's Closest Friends and Most Inveterate Enemies
Series 3: Trees and Other Plants Useful to Man (Illustrated by J. Marion Shull)
Series 4: Some Common Wild Flowers
Series 5: Among Our Feathered Friends
Series 6: Native Wild Animals
Series 7: Life In and Around the Water
Series 8: Insects, Helpful and Harmful (Illustrated by Lynn Bogue Hunt and Doris M. Cochran)