I don't think there's any specific written prep that offers any value. You know your weaknesses, take those and gets a different CFI's perspective on them. Then go learn that, and go back and explain it to him/her. That includes any areas you missed on the written. Watch a mock checkride on the internet. There are a couple that are free. They aren't all the same, but it'll give you an idea. Be expected to answer judgement based questions as part of theoretical scenarios. They're not looking for rote answers, they're looking for understanding. Make sure your review the aircraft logbooks for all the appropriate recurring checks - and make sure they're done. Review your own logbook, to, and be ready to demonstrate all the minimums. Get plenty of sleep, don't stress it.