No sensible seller will think anything of it if you open an item not received dispute after about 4 weeks, you can still give it another 2 weeks before escalating to a claim.

Parcels from the UK are as relaible as a lotto number picker, I have had parcels that have arrived in three days and others that have taken six weeks. If it is something that is going to stress you out then don't buy fr4om overseas.

If you purchaser on a well known book site in the UK you may get your books in a few days as they are using Royal Mail or they may be posted from their German distribution centre and then they will take anything up to three weeks.