I really love the Heinemann “InfoSearch” science series; unfortunately, the books are no longer in print. You might be able to pick up some of them used, though, as through Amazon or eBay.
Each book is 32 pages and filled with solid information, simple experiments, and historical context. The pages are laid out nicely in full color.
If I find another in-print series I like as well I’ll write about it; for now I feel fortunate that I was able to collect ten books from this series. My five-year-old loves them.