The Killing Kind, the third book in the Realm of Tah’afajien series is a prequel that happens over a decade before the beginning of Not Gods But Monsters. Book 3 provides context, not only for what transpires during books 1 and 2 but for the series' eventual endgame. Think of it like The Phantom Menace (but, you know, better (hopefully)). Or like when they decided to film the Hobbit movies well after the Lord of the Rings.
It has always been my intent that the series was to be arranged as three sets of two books, with each pairing transpiring at different points across the 15-17 year timeline (see infographic to the right).
The Killing Kind (Book 3) - 2059 BAE
Book 4 - 2061-62 BAE
Not Gods But Monsters (Book 1) - 2070 BAE
A Prison of Flesh (Book 2) - 2071 BAE
Book 5 - 2074 BAE
Book 6 - (approximately) 2074-76 BAE
What order should I read the series in?
It depends on you. Have you already read Not Gods But Monsters? Maybe this will be your first title?
Realistically, they can be read in two orders: Release and Chronological. The Killing Kind is written with the intent that the reader has not yet been introduced to the series. If you have, though, then you'll see a number of references and call-backs that are familiar.
What about book 4?
If you want to read the series chronologically, you can read The Killing Kind and go straight into Not Gods But Monsters without Book 4, if need be. While Book 4 does answer how certain characters ended up where they are in Not Gods, Book 4 does more to lay down groundwork for Book 5.