Apple are again doing their limited time First in Series giveaway, this time they have got Crime & Thriller as the genre.
The link is for iTunes, but you should be able to access the giveaway through the iBooks store on your iDevices.
also in the past Amazon has price matched and given away the Kindle versions. I've not checked. UPDATE: see links in posts #16 & 17 many thanks for the all the links, you've got some patience