They were mostly published in the German and Austrian magazines and newspapers. Even then, not too many. I wonder where the family archives ended up? I suppose, if the material wasn't handed over to the main Hessian archives, it would be with Assen's descendants?
I don't think there was too close a relationship, at least not one mentioned with much frequency in the memoirs of Victoria Milford Haven or the published memoirs of Princess Marie. The latter mentions Johanna only in passing and without much warmth. The family also lived in Austria so there was a geographically separation as well, especially post-WW1. She remained in Vienna even after WW2 as did her son. Perhaps this morganatically founded family had issues with an opera singer? At least, Julie Hauke came from a noble background.
I think Marie mentions her somewhat in connection with the burial of Alexander in Bulgaria? If I can remember, it had to do with her agreeing for him to be buried as their former ruler, knowing that she wouldn't be buried with him.
You can see the graves of Johanna and Zwetana here:
http://royaltyguide.nl/countries/austria/graz/friedhofleonhard.htm