This is what we have collected the last 2 years: (my boys are almost 5 and 7 and they use the first bug book one all on their own – they Love bugs, butterflies, beetles, etc and this book has it all:
National Wildlife Federation Field Guide to Insects and Spiders of North America (this is our favorite!) Nice waterproof jacket too.
The Backyard Birdsong Guide (for our region) plays their songs also; 2nd favorite, and is used 1-2x wk
We also have a nice easy one on flowers by color: A Falcon Guide: Rocky Mountain Wildflowers that is great when we go on a hike
A Golden Field Guide Birds of North America (this is also used by the kids about 1x week or so) Helpful to add tabs to it.
There are others that we just don’t seem to use much unless I make a concerted effort to make it into a study: Plant Guide, one on trees, I don’t have one on mammals of our area, but we can find lots of fun animals picture books at the library for that.