Claude F. Bragdon through his contributions to The Chap Book, particularly as an illustrator. If the name sounds mildly familiar to some of you, it may be because I appropriated one of his illustrations as the skeletal servitor in the "Minions" section of the expanded version of Petty Gods.
The other day, while perusing some theosophy and metaphysical texts at archive.org, I came to learn that Mr. Bragdon was also an advocate of theosophy and a writer of spiritual topics. In that measure, I'm sharing a link today to his 1913 work, A Primer of Higher Space (The Fourth Dimension).
From the Foreword: The Fourth Dimension of Space is a memory-haunting phrase, often heard, yet little understood. In this slender volume the author has attempted to explain what is meant by it, and to usher the reader into some of those fascinating halls of thought to which it is the door. Although the exposition is along known and already conventional lines, it is hoped that there is enough new and original matter to justify the book, and enough simplicity of presentation to justify it's title, A Primer of Higher Space.