The IMSLP has many out-of-copyright pieces of music as well as interesting articles and books:
I happened across this one:
https://imslp.org/wiki/Organ_Stops_and_...e_Ashdown)
Organ Stops and Their Artistic Registration (Audsley, George Ashdown)
Another such is:
https://imslp.org/wiki/Organ_Registrati...Ellsworth)
Organ Registration (Truette, Everett Ellsworth)
Though this one seems more technical in nature showing the various waveforms a particular pipe will produce - a quick glance brought me to look upon what seemed to be a square wave. From my understanding of synthesizers I would say it's that of a reed stop producing a distinctive clarinet-type 'hollow' sound of odd harmonics - but what do I know???
Enjoy...