Accounting entry for purchase of books or journals

There is a manufacturing business of pipes where sometimes (and its only sometimes like once in a year or two and only when its required) they buy journals or books from organisations that publish conformity standards. So under which head of expense would this transaction go? Membership and Subscripttion may not be the right ledger here because the purchase is not of a service but rather a product. Publication purchase expenses would be the ideal one I believe. Please guide me.