What a treat to read your post, Natascha! I've become intrigued by vertical writing recently, too, and am also struggling to find information. I was given an old copybook by H.W. Shaylor: Vertical Round Hand Writing. I love the stylized writing on the cover (I'll attach a photo), but the contents of the book show a much simpler hand, for young children.
In my hunt for more information, I've come across a number of online resources, most of which are geared toward teaching children. Here are some books available as downloadable pdf's:
http://catalog.hathitrust.org/Record/100312999?type%5B%5D=all&lookfor%5B%5D=Vertical%20writing&ft=The Volpenna Vertical Writing Lessons: a complete manual for learning and teaching this original system. R.K. Row, A.F. Newlands
http://library.duke.edu/digitalcollections/eaa_A0566/American System of Vertical Writing, American Book Company, 1894
https://archive.org/details/cihm_92542Public school writing course: vertical system, Ontario Department of Education, 1896
https://archive.org/details/cihm_93933Gage’s Business and Social Forms, Vertical Writing, R.H. Eldon and Alexander Clark Casselman, 1896
https://archive.org/details/teachersmanualt00newlgoogTeachers' Manual to Accompany the Natural System of Vertical Writing, A. F. Newlands, 1907
https://archive.org/details/publicschoolwrit00newlPublic School Writing Course Vertical System : Junior Fourth No 5 by Newlands, A. F. (Alexander F.); Row, R. K., 1896
https://ia902605.us.archive.org/10/items/theorypracticeof00jackrich/theorypracticeof00jackrich.pdfThe theory and practice of handwriting; a practical manual for the guidance of school boards, teachers, and students of the art, with diagrams and illustrations, Jackson, John, 1894
I've found reference to many other vertical writing copy books, but they may only be available in hard copy. There's one that might be of particular interest:
Spencerian penmanship : vertical edition, Platt R Spencer, 1896
I'm guessing this volume might have some more ornamental forms of vertical writing. I haven't yet seen a copy.
Schin kindly passed along information she had gleaned from Joe Vitolo. Vertical writing was apparently short-lived and did not catch on. It was, for a few years, highly touted as being a more efficient, more legible, and more easily learned form of writing, but then faded from widespread use. Slanted writing reigned supreme!
I'm eagerly looking forward to information and resources which others may have.
LindaR