There are many theories & interpretations, but relatively little evidence to support most of them. Of course, there are no written sources from the Ubaid period to support them.
You are absolutely right that there hasn't been a great deal of published material on the subject. However, a good, and relatively recent (2006), paper on the subject is A ...
I have found full list of proto-cuneiform signs (very large file [93 MB]), with very detailed glyphs together with their meanings sadly meanings seems to be all missing. Though there is no author information.
Also as you suggested; A. Falkenstein, Archaische Texte aus Uruk (Archaische Texte aus
Uruk 1; Berlin-Leipzig
from Cuneiform Digital ...