Hi, I've just seen that the Book 0.5: Zombies vs The Living Dead is currently not free on the UK Amazon store. Amazon don't allow authors to permanently set the price to 0.00, but price match against other stores, and reserve the right to raise the price as and when they wish.
I'm not sure if this is a temporary glitch, but you can find the book elsewhere for free.
Whilst I'm appreciative of the £0.20 a copy I get from each sale, I really think you'd enjoy Book 3 more if you've read this short story. You're certainly going to have want to have read it before Book 4: Unsafe Haven and... (though I'm not confirming this as definite until I've got the covers)... Book 3.5: Svalbard, Book 4.5: AWOL, Book 5: Return and the absolutely final book in the series Book 6: The Blood Red Line.
(I'm also planning a separate 1 novel, 1 short story spin off set in the US, but more on that closer to the time.)