When you're sitting an exam, it's OK to be nervous, but try not to panic. An open-book exam is another form of intellectual analysis. It is not very different from preparing an assignment at home, except that time is more limited. See our guide to Surviving Exams.
You might feel too nervous to eat, but your brain needs fuel! If you don't eat, halfway through the exam, you'll find yourself feeling tired, hungry and losing concentration. It's also a good idea to take some bottled water into the exam room.
If you're sitting a take-home exam:
Before you start the exam, make sure you read all the questions carefully and understand what you are being asked to do. Look for clue-words in the questions.
Answer the 'easy' questions first; starting with the 'hard' questions slows you down and can leave you with insufficient time to address the others that you can do well in.
Don't copy long chunks from the text or waste time quoting extensively. Use very few quotations–your own analysis is more important. Use the time and information to build a well-constructed answer.
With lots of materials available, it can be tempting to over-write and waffle on. Answer the question, and aim for concise, accurate, thoughtful answers that make good use of supporting evidence.