The issue with our digital library is that the search there is not indexed, and most old German texts are not searchable either [1]
You can search for "Allgemeiner Deutscher Sprachverein" or "Atlas der deutschen Sprache" or similar, you'll find references to it but not the scanned books.
Welcome to the age of Digital Amnesia :'( [2]
Maybe the dossier about the burned books of WW2 is a good start to find things about it, but it's kinda hopeless without a search index. Alternatively there seems to be a lend-able copy in the library of Dresden [3] and [4]
