Following are a few books that my parents gave me when I was so young that I don't ever remember not owning these books. Certainly I had all of these books before the age of 10 or 11. I read each of them multiple times growing up.
First is Find the Constellations, by H.A. Rey, who is also well known for the Curious George series. Rey's intuitive approach to finding the constellations and his charming illustrations and explanations make his works on the subject valued possessions. My dad and I would often walk outside into the night and identify the constellations.
It wasn't long before our family added H.A. Rey's The Stars: A New Way to See Them to our library. Find the Constellations is really the children's version of this book, and it is an excellent one. However, the full version is even more valuable. This previous post explains how Rey's revolutionary new method of outlining the constellations is far better than anything that came before.
Several previous posts have also mentioned the wonderful illustrated version of Norse mythology by Edgar and Ingri Parin D'Aulaire, Norse Gods and Giants. While it appears to have a new title, this excellent book is still available for the young mythologists you know.
The companion version to the D'Aulaires' book of Norse myths is their Book of Greek Myths. I spent so much time with both of these books that I knew just about every word, and certainly every illustration, by heart.
The 1967 Life Magazine publication entitled The World We Live In was another favorite when I was very young. It is full of wonderful illustrations and some outdated scientific theories. I would spend hours in it, especially the dinosaur sections. One of my cousins, who is older than me by five years, used to call it "The World We Do Not Live In," which I always thought was hilarious.
There were many other books that were big influences on me from a young age as well. I remember that the books of Thor Heyerdahl were always around the house, as was the autobiography of Danish Arctic explorer Peter Freuchen entitled Vagrant Viking. All of these are excellent reads for young and old.
