While this is nice and easy to do with Smashwords (which populates iBooks, Kobo, Barnes & Noble and so on), Amazon doesn't support free books unless you've enrolled in KDP Select, which limits you to selling only on Amazon. So - this is a million dollar question for authors - how do I make my books free on Amazon for Kindle without enrolling them in KDP Select?
First you pop over to Amazon and set your royalty rate to 35%. This is important, because at the 70% royalty, you undertake to Amazon that you won't sell elsewhere at a different price. So when you start messing about with prices on other platforms, Amazon can (and has every right to) get a bit tetchy. At the 35% royalty, there are no such restrictions.
Now you set the price to zero in Smashwords and wait for that change to impact iBooks and the other stores served by the multi-publishing marvel. Amazon picks this change up and at its discretion will match its own price to that of iBooks, Kobo and Barnes & Noble. If Amazon's price crawlers don't pick it up after a few days, there's a little button against the book's page on Amazon that allows you to request a price match by providing a link to the competitive store where the other pricing (ie: free) is offered. Note it doesn't work if you provide a link to Smashwords. It has to be a retailer.
My first, silly, book Space is, and always has been, enrolled in KDP Select simply because I wanted to play around with the platform. KDP Select allows you up to five days' free promotion every three months but does have that terrible drawback of only letting you sell via Amazon. And only five days' free promotion isn't quite enough to really make an impact, in my humble opinion. Olives is now perma-free so I can provide a sampler to an increasingly skittish and wary book buying public. And if they like that, they can buy Beirut - An Explosive Thriller for $0.99 and Shemlan - A Deadly Tragedy for just $1.99!
Does 'free' work? Well, in the first 24 hours, I've shifted over 500 books. Will it win me readers, reviews, accolades and plaudits? We'll see. I'll let you know how it goes if you sign up for my occasional freebie, hints and stuff emailer... See what I did there?
Oh, by the way, A Decent Bomber and Birdkill will still cost you a reasonable $2.99. You ain't getting them for free...
PS: Amazon also picked up a momentary blip on Beirut's pricing as I was playing around and made THAT free too in the US, although not the UK. For some reason, downloads of Beirut have massively eclipsed Olives and it's now #2 in the Thrillers & Suspense, Espionage listing of the Kindle store. Which is nice...