In the original book, there are forty two one movement Rabboni Sonatas, aimed at exploring sonority and developing technique. He was clearly a great admirer of the human voice and its similarities with sound production of the flute. These sonatas certainly encourage the performer to approach the task in hand in a vocal manner.
However, it transpires that Rabboni also had magpie tendencies, as many of these pieces are arrangements of the music of other composers (an example being No. 13, which is in fact taken from a sketch of the slow movement of an oboe sonata by Donizetti, but rewritten up a fourth!). I have yet to find any of the original piano accompaniments.