Hi Jenafur, I have a chemistry-mad 6yo DS. His latest love is actually particle physics, which if you think about it is an obvious next step after loving the periodic table... He's enjoyed Russell Stannard's Uncle Albert and the Quantum Quest just recently (mostly about wave particle duality really); not a book, but we all watched the Great Courses particle physics course together:
http://www.greatcourses.co.uk/ttcx/coursedesclong2.aspx?cid=1247[there is a US site for this but I've lost it; and in case you don't know, never buy from this company except in a sale, everything comes round regularly!] - I mention this since it might not be obvious that it's accessible to a 5 or 6yo, but it really is. (We do also have their High School Chemistry course, but have not got into it - we were both very put off by the opening lecture that goes on and on about how you of course think science is boring, when neither of us does think that!)
Back to books, I also recommend the Usborne Illustrated Dictionary of Chemistry - lots of stuff in it, clearly presented with loads of illustrations. (It's not a dictionary really.)
A molecule building kit to go with this is fantastic; DS has an Orbit set which keeps getting added to.
Sorry, only two books in amid a lot of stuff that wasn't what you asked for!
