Mathers rests his feminine Holy Spirit on a line of the Sepher Yetzirah, which he renders "One is She the Spirit of the Elohim of Life." Kalisch's edition of that book, also in the reader, prints the same Hebrew in Chapter I, section 8, and shows what the feminine word is doing there. It opens a count of the ten sefirot: one, the spirit; two, air from spirit; three, water; four, fire. Every numeral in the list takes the feminine form, because the noun being counted is sefirot, and Kalisch simply numbers the line as the first item. The feminine form is what the count requires whatever the gender of spirit; it cannot carry the creed Mathers builds on it.
contrasts as a translation with Sepher Yezirah: A Book on Creation — trans. Isidor Kalisch (1877)