Nikolaj Schlej has written the first of a series of articles on NVRAM file formats:
“NVRAM formats of UEFI-compatible firmwares”
It is in Russian. If you don’t read Russian, there are many C structs and colored screenshots that are self-explanatory, and auto-translators (like Google Translate) work pretty well.
If you’ve not been watching UEFITool NE recently, there have been lots of checkins for NVRAM formats.
Nikolaj is also looking for some NVRAM formats for testing: