Erler published two volumes on Saar -- one for Saar generally (Vol. VI) and one for the municipal revenue stamps (Vol. XIII).
Here's the excerpt from Vol. VI.

I'm not aware of anything more recent, but I honestly haven't stayed on top of it since I don't actively collect Saar. I don't see any newer revenue publications listed on the website of the ArGe Saar (Saar study group of the BDPh).