Johann Martin Schleyer published the first sketch of Volapük in 1879 in a rather obscure catholic newsletter named Die Sionsharfe. This newsletter isn't easily accessible, as far as I know. Is this first sketch available as a reprint in some more accessible work?
To my big surprise the relevant issue of Sionsharfe is now digitised and freely available from Digitale Bibliothek München. The first sketch of Volapük at this moment still named only Weltsprache or Allsprache, the word Volapük itself can be found a few month later, occurs in Nr. 35, Vol. IV (1879) Beilage as a supplement.
What a fascinating document!
Following issues of Sionsharfe also contain shorter or longer fragments of Volapük, unfortunately the digisat ends with Nr 48 (1880), there are four more years missing.