Also I forgot a couple of others that have humor which are the Grey Mouser series by Fritz Lieber and The High Crusade by Poul Anderson. The High Crusade is one of my favorite stories.

When I was in the 4th grade I brought home Heinlein's Red Planet from the school library to read. I didn't even know what science fiction was but I went back and read everything by him I could find. Then I discovered a Andre Norton a lady who wrote the Ross Murdock/Time Traders series.