If you're a fan of Seanan McGuire's Incryptid Series you're going to buy this book regardless of reviews. If you haven't read "Midnight Blue-Light Special" which starts the series, I'd recommend starting with that. Yes, "Half-Off Ragnorak" can stand alone, but the world building does use a bit from previous books.
In the first two books, the narrator was Verity Price. This time around her brother Alex gets his story. I was a bit concerned that we'd just get a male version of Verity (such a strong narrative voice), but Seanan McGuire makes Alex quite distinctive. Quieter but just as daring.
As always, the charm of the series is the cryptozoological world with its creatures and characters, and the fast pace. To be honest, I felt the plots was less than the characters and their adventures in this strange world. It existed to let them romp through it, but that was fine with me.
As an Australian, I was intrigued by mentions of our alternate reality. So much of the weirdness is true ;)