While I don't have casebooks, I recommend the following to keep your German sharp:
1. Conversational: Download the app HelloTalk. It pairs you up with people that are opposite to you (i.e. learning your language, but native in English) and you chat via text, audio, call etc. It has awesome features (such as edit functionality so the other person can edit your messages for correctness).
2. Vocabulary: Use either handwritten flashcards and/or the app Quizlet. Review these whenever waiting (i.e. on the bus, cooking, on hold, etc)
3. Listening: Download English podcasts and listen while you sleep. I.e. put the phone next to your bed, at a soft enough volume that you can sleep, but loud enough that you cna hear the words. You'll be amazing how your listening improves.
4. Dedicated/tailored training: Go to www.italki.com. It's the PrepLounge for languages