I have heard the “Hollow Reed” song many times, used in devotional groups. The first two lines, or so, are often used as a prayer. Each time someone asks for the source of the lines, it seems apparent that its source and author is “The Book of Hearsay.”
Be that as it may, the entire writing can be located with research. A hollow reed was a theme used as a symbol of how human beings are plaintive and cut off from their divine origin, most famously used by Jalalu’d-Din Rumi in his Mathnavi. The use of the phrase “pith of self” appears to be thematic of Merkabah mysticism, early Jewish literature during the Christian era.