User blog comment:SayuriDarling/The Sword of Summer Official Discussion/@comment-24055022-20151020030254/@comment-5984859-20151021192946

I think the name of the third book is the Sword of Tyr. think of it:

Sumarbrander is formerly the weapon of Fenrir/Surt's arch-enemy, Frey.

Mjolnir is the weapon of Jermongand's (how to spell that name?) arch-enemy, Thor.

Tyrfring is the weapon of Garm the hell dog's arch-enemy, Tyr.