Should I book Air Berlin in advance given the ongoing take over?

In general you should avoid making any business with an insolvent company. If there is any problem which results in them getting in debt to you (like owing you a refund or compensation), they are actually forbidden from paying you directly. You will be put on the list of creditors and won't get a cent until the liquidation is over. And when there is not enough left after they paid their debts to those who have priority (and among those is anyone they owed money to before declaring bankruptcy), you might not get all the money they owe you.