James Derrick
Hi,
The ground truth is an excellent bookshop as Alnwick Station closed in 1968, however building=train_station might fit as the architecture is unmistakable. It might also be useful to add lifecycle tags such as historic:status=closed or abandoned:railway=station to acknowledge the pre-1962 status.

Note - the current ECML station is Alnmouth for Alnwick ALM, and although the Aln Valley Railway is restoring the connecting line, the chances of reconnecting the bookshop are almost zero.

Barter Books has been restoring the building for a very long time:
https://www.barterbooks.co.uk/html/About%20Us/The%20Bookshop.php

It's been a while since I visited the bookshop so can't give a definitive split between the two shop ways, but suspect it's one entity (apart from Jewson).
Happy Mapping,

James