I was fortunate enough to find and be inexplicably captivated by the 1923 edition of "A Primer of Higher Space" (originally published in 1913) at the exact time of my life when I needed it. Were it not for dating a rare books librarian at the time, I would have never been able to "bend the rules" and sneak this book out to read, and my life would have turned out very differently.
There has never been and probably will never be a book which is more important me, and I don't think I can fully explain why. "A Primer of Higher Space" is a very simple book of far less than 100 pages (most of which are diagrams) that describes basic ideas about multi-dimensional space. As was the case with many of the late 19th century / early 20th century "esoterics," mathematics and science was presented along with spirituality and philosophy and Bragdon opened new doors of thought (regarding religion, death, time, knowledge and being) for me as I read this book for the first time. The world would be so different today if those branches of thought hadn't diverged so profoundly, and it is encouraging to see the apparent re-convergence in recent times.
Years after my first reading of this book, I found the exact edition on eBay and I find myself turning to this book again and again. Since then, I've found quite a few Claude Bragdon books from the 1920s and 30s and have enjoyed them all.