The Childcraft How and Why Library was first published in the 1930s. The World Book Encyclopaedia company was the holder of the Childcraft imprint.
The edition I have was published in 1969. It consists of 15 volumes, each grey in colour with a different colour on the spine.
I bought mine from a local op shop (charity store). I wish I’d had them when my children were younger. They have such a huge variety of content ranging from poetry and stories, to facts and figures and things to make and do.