As a German I've learnt English as a foreign language at school. Not using the English language every day, a lot of grammar knowledge gets lost. Every time looking into a German standard English grammar? That might be too lavishly. But using the Oxford A-Z Grammar is an easy and successful way! Short answres sum up all you want to know. And coaching German pupils in English, I could quickly find answers to their questions.
This grammar does not substitute a complete grammar while learning English for the first time, but it is a very helpful little book for advanced learners - it will accompany me on my travels to the UK.