[shakna]

The two big ones for discussing Germanic languages and their inheritance would probably be:

"From Proto-Indo-European to Proto-Germanic", Ringe.

And the simpler "Lexikon der indogermanischen Partikeln und Pronominalstämme", Dunkel.

Both use "n̥-s-ero-", though in the more traditional /ˈun.se.rɑz/ form.

[z500]

Oh I've actually been meaning to get around to reading that first one. I'll have to crack that open.