Current work on 2 Corinthians 1 means that I am having to spend some time in the German secondary literature (a wonderful article by Otfried Hofius on 2 Cor 1.3-7, plus the commentaries by Windisch and Bachmann which still repay consultation). I have to say my German felt distinctly rusty as I sat down with these texts, so the other week I cam across (can't remember where, sorry) a recommendation for April Wilson's German Quickly: A Grammar for Reading German, and it has now arrived and I have to say it looks excellent. I wish it had been around in my first year of postgraduate studies (in fact the book is the product of Wilson's teaching of 1st year humanities graduate students in Chicago and her own MA is in religion and literature).
The lessons are clear, the examples relevant, the progression well organized. If you need to learn some German for your own research, then the best thing is to go and study there for a year or more (this used to be the standard path for young Baptist NT scholars – alas no more), but failing that, Wilson's book is a really good way to get enough information on board to begin to start reading the great untranslated for yourself.