No.18 is The GiantGolden Book of Biology: An introduction to the Science of Life, illustrated by the wonderful Charley Harper in 1961. I have only ever seen this book on design blogs, but the illustrations and colours look fantastic. These images are from Grain Edits Flickr group which has loads of great pics from the book.
I would love to own it! Copies do come up on Ebay quite often and there are always some for sale here, but at prices from $599 it's a serious purchase, so I'm still hoping to find a copy at a boot sale!