Have you tried the brainwaves? My DS LOVES them. They have a lot of fact but also are presented in an amusing cartoony setting. It's like like the magic school bus books squared and without main characters. It's from DK publishing, they do pretty decent encyclopedias. Fiction rather than non fiction was the Stephen and Lucy hawking series George and the secret key to the universe. My only complaint with it is that the main character has some ineractions with bullies and stuff appropriate for a 10 year old but not so much for my 4.5!!! But I enjoyed it as well, we knew a lot of the science discussed so seeing it applied really appealed to my son and the stuff we didn't know was explained really well. Also someone elsewhere mentioned the uncle Albert books but I haven't found them yet.
Hawking book Brainwaves 
