Currentely not available in English it seems, but I got a Dutch translation of this book. I knew the sufi-master Hazrat Inayat Khan (1882-1927) and I expected a sufi approach to the Gathas of Zarathustra, but “Gatha” just means something like “teaching” so this was not the case. Khan came from India and shows knowledge of not only Islam (of course), but also Hinduism, Buddhism and Christianity. The book shows a spiritual path by explaining everyday-life things. It is a practical book, but not in the sense of exersizes. I was surprised to see many similarities with the modern Theosophical teachings, the approach seems to be similar, the interweaving of Western and Eastern doctrines can be found here as well. The text is very readable and therefor a good introduction to sufism for the Western man. Two quotes to close off:
“Which faith is taught by a sufi?” “No faith, but he helps the student to look for and find his own faith.”
“sufism is not a faith but a viewpoint”