I'm looking for literature about multi/pseudo graphs and/or homomorphisms between them. Here by multi/pseudo graph (the terminology is not commonly agreed upon) I mean a graph that could have multiple edges between two vertices (which don't have to differ), also I would prefer literature that is not confined to finite cases. I've looked into the references given on the Wikipedia page on multigraphs, but just as I expected these cases are just mentioned shortly, as it happened in all graph books I've encountered yet.
My local university library doesn't list any books or other references given the key words "pseudo graph" or "multi graph", so I'm at loss here.
EDIT: The graph theory book of Bondy and Murty from 2008 suggested in the comments is a good start, as well as the graph theory book of Wilson from the 70's I've found in the mean time. But both of them only handle isomorphisms, not general homomorphisms. I still need literature about this.