So, I’m looking through the stack of Howard-related letters and got curious. The letter above appears in The Collected Letters of Dr. Isaac M. Howard; the original is housed at the Cross Plains Public Library. Who, I wondered, were John M. Watkins and Dr. Anna Kingsford, and what kind of books was Doc Howard looking for?
Watkins, it turns out, was a bookseller and publisher of some rather interesting sounding titles, including The Sacred Magic of Abramelin the Mage, Christos: the Religion of the Future, and Essays on Alchemy. “John M. Watkins / 21 Cecil Court, Charing Cross Road / London” is listed as publisher of Addresses and Essays on Vegetarianism (1912), by the aforementioned Dr. Anna Kingsford. The long “Biographical Preface” from that volume is presented here.
Another Watkins-published book, The Living Truth in Christianity (1915), by Bertram McCrie, is described on a Kingsford website as follows:
This little book was written as an introduction to the message of the two soul-prophets – known in this age as Anna Kingsford and Edward Maitland – namely, to the recovered New Gospel of Interpretation, the doctrine which they were instrumental in restoring to the West, where the Christ is risen again from the sepulchre of historical tradition, to live and reign in the undying soul of man.
For more than you probably want to know about Kingsford, there is “A Brief Biography” here. It begins with the following:
There have been few lives as incredible as the life of Dr Anna Kingsford, both in the sense of being remarkable and also of being slightly inconceivable. A theosophist, self-proclaimed prophet, feminist, vegetarian, mother and woman of much personal charm, Kingsford is impossible to classify. This may explain her conspicuous absence in contemporary Victorian studies. As a vegetarian, she makes brief appearances in texts about vegetarianism. As a theosophist, she is delegated a paragraph or two in theosophical studies. Either way, further studies of Kingsford could tell us much about Victorian occultism and the place women carved for themselves within it.
The Theosophy angle reminded me of Jeff Shanks’ recent article in The Dark Man (vol. 6): “Theosophy and the Thurian Age: Robert E. Howard and the Works of William Scott-Elliot.” This is outside my area of expertise, but given the 1925 date on the letter above, one wonders if Kingsford’s The Perfect Way (1882), a collection of her mystical visions and dreams, made it onto Howard’s reading list. The book is mentioned on Kingsford’s page at the “Mysterious People” website: “At the time  The Perfect Way, which explored the deeper Mysteries of religion, was extremely well-received and regarded as the most important modern book of esoteric wisdom and mysticism ever published.” Kingsford’s relationship with Theosophy is also discussed.
Watkins Books, by the way, is still around and still located at 21 Cecil Court; their website says, “Watkins Books is an esoteric bookshop in the heart of London. Established over 100 years ago, we are now one of the world’s leading independent bookshops specialising in new, second-hand and antiquarian titles in the Mind, Body, Spirit field.”