The first format of the Young People’s Library has a characteristic spine as noted here:

Book #12 Mother Goose’s Rhymes and Jingles is the only exception I have seen until now.

Here is Book #6 Child’s Life of Christ with a blank spine. Note the usual spine on the same title.

Both of these books are the earliest 1895 format based on the ads seen in the front.

This blank spine book may be just a print error or it is possible it is the earliest Format 1 style and that as of yet no other blank spine books have been seen.