Important bibliographer of mathematics literature for the time 1445-1926.
His catalogue of mathematical manuscripts ("Allgemeine Mathematische Bibliographie") contained ca. 200.000 items, when he died.
Catalogue progress descriptions as given in "Jahrbuch":
1885,
1897,
1900,
1910
Valentin's activities are described in an article in "Jahrbuch über die Fortschritte der Mathematik 58 (1932), p. 44-45 (local copy)", which refers the content of a talk given by R. C. Archibald on the ICM 1928. Cf.: R. C. Archibald: Georg Hermann Valentin (1848--1926). Atti Congresso Bologna 6, 465-472).
Plans to publish the catalogue failed because of lack of money and because there was no suitable publisher available. The catalogue stayed - as a collection of handwritten cards -- at Preußischen Staatsbibliothek in Berlin and was (together with the library) destroyed by a fire in 1944, probably caused by a bomb attack.
Apparently there are only fragmentary copies.