The first three titles in James Benn's Billy Boyle series (which has one of the best series premises around, IMO) are on sale right now in the US and UK. As I've mentioned before though, the rest of the books in the series are very expensive and rarely go on sale. So if you like the first three, you'll have to go to your local library (or not-so-local library - luckily MR has a thread for that...

) or end up spending too much!
Billy Boyle, #1, $2.99/£1.89
Kindle US:
https://www.amazon.com/dp/B004HYHAGM
Kobo US:
https://www.kobo.com/us/en/ebook/billy-boyle
Kindle UK:
https://www.amazon.co.uk/dp/B004HYHAGM
The First Wave, #2, $1.99/£1.42
Kindle US:
https://www.amazon.com/dp/B004HYHA8A
Kobo US:
https://www.kobo.com/us/en/ebook/the-first-wave
Kindle UK:
https://www.amazon.co.uk/dp/B004HYHA8A
Blood Alone, #3, $3.99/£2.59
Kindle US:
https://www.amazon.com/dp/B004HYHA9Y
Kobo US:
https://www.kobo.com/us/en/ebook/blood-alone
Kindle UK:
https://www.amazon.co.uk/dp/B004HYHA9Y