Feature Request - For C.C.BUCHNER add click-and-teach.de

Hi there,

it would be great if you could add the possibility to download from [click-and-teach.de](https://www.click-and-teach.de) as well as from [click-and-study.de](https://www.click-and-study.de). My login credentials for click-and-teach also work for click-and-study, but my books are not listed there.

Cheers