Hi Sharon! What a fun mystery that I finally had some time to dig into.
I searched the Library of Congress's own books by title (as you likely did as well) and also didn't find anything. Searching the web broadly for "book of forty-five mornings" AND "kipling", I found several references to this as a book that Kipling planned but never wrote. I also found additional advertisements as well in books of the period. Fascinating!
The sources I found were:
- Kipling, Rudyard, and Thomas Pinney. The Letters of Rudyard Kipling: Vol. 1. Basingstoke [etc.: Macmillan, 1990. Print. (page 6, in footnote 2)
- Crook, Nora. Kipling's Myths of Love and Death. Place of publication not identified: Palgrave Macmillan, 2014. Print. (page 193, in footnote 44)