PRE-Order

a pre-order is always sensible because there is often only a limited number of items, so you are on the safe side that when the item is not published the item is sold out in advance. The order does not have to be paid immediately, if you want you can pay the order immediately or only after the article has appeared.

Combine deliverable items with PRE order?

an order is always dispatched when all items from the order are available. Everyone has to decide for themselves whether to pre-order the e.g. in 5 months appear in one order combined with immediately deliverable items. However, you can call up a partial delivery at any time, then postage costs for the partial delivery are added.