The Survivors Book I - IV are designed to be read sequentially.
The Survivors: In A Heartbeat can be read any time after Book I.
The Survivors: Enigma is a non-sequential alternate history
that may be read at any time, but will be most enjoyable after
completing the main series.
Recommended Reading Order
A tale of love, hope, and humanity set against the backdrop of post-apocalyptic New Zealand.