Thomas Watson Ball was an art director at Harper's when this edition of Spun-Yarn was issued. He did use seahorses as a spine element a few years later on Light Freights, and there are some similarities with Ball's lettering styles, but there is not yet enough evidence to give this design a firm attribution. If you have seen any similar designs, please do comment.
[unknown artist]
Spun-Yarn
by Morgan Robertson
Harper, 1898
Margaret Armstrong
[signed with MA monogram]
Little Rivers
by Henry Van Dyke
Scribner's, 1895
Armstrong had previously done work for Harper's, but they did not allow her to use her monogram. Below is an example that is not in G&E *. It is clearly her work, both the lettering style and the design in gold and "silver" (white metal). Thanks to John Lehner for pointing it out:
Margaret Armstrong
[unsigned]
The Christmas Hirelings
by M. E. Braddon
Illustrated by F. H. Townsend
Harper & Brothers, 1894
* Gullans, Charles and John Espey. Margaret Armstrong and American Trade Bindings. UCLA, 1991.