The MERLIN catalog lists a wealth of language learning materials. A general subject search for "German Language" will retrieve hundreds of books. To see what is available in electronic format, click on Limit, and choose Ebook as the Material Type.
Add the following sub-headings after German Language to get more specific results:
The following databases list research on linguistics and language pedagogy. See also the MERLIN catalog, using the broad subject heading "German Language", or the more specific "German Language Study and Teaching".