As the experts are continually telling us, book collecting is not philately. Production errors and eccentricities do tend to devalue books.

The only Robin Hood book published by A&C Black I can find is "The Story of Robin Hood and his Merrie Men" by John Finnemore, illustrated by Stoddart, 1925. So I am presuming that they had some left over, and they printed some on the other side of the paper.

As you no doubt know, this book was in print over a long period, and is a very common title, so has no great rarity value.