Margaret Neilson Armstrong (September 24, 1867-July 18, 1944) was a designer of cloth bookbindings who produced over 250 cover designs. She was also an accomplished botanist, editor, and author.
Biography on Wikipedia: https://en.wikipedia.org/wiki/Margaret_Neilson_Armstrong

Margaret Armstrong’s cover design and lettering for Twenty Centuries of Paris
Written by Mabell S.C. Smith / Published 1913 / Image from Women of Graphic Design

Margaret Neilson Armstrong covers and illustrations on The Met website: https://bit.ly/3kBYK6G

Further reading:
https://libapps.libraries.uc.edu/exhibits/margaretarmstrong/
https://rarebookschool.org/2005/exhibitions/armstrong.shtml
https://rbscp.lib.rochester.edu/3351