Dear Beatrice,
You want to measure and map similarity between documents (or another variable linked to your documents) based on the proximity between the references mentioned in each document?
If yes, you just need to perform a Network Mapping analysis, using a _het metric.

1. In the nodes panel: choose two different fields
2. In the edges panel: choose a proximity measure that feet to heterogeneous networks. Cosine_het should feet to your need.
3. In the edges panel: chose if you want to map both entities or only the first ones (linked through the linkages of the second variables). In your case just let the default parameter, cosine should fit
4. In the Network Analysis & Layout panel: choose Louvain resolution community detection algorithm

I hope it helps
L