JD Robb's (Nora Roberts) first 4 In Death books are currently on sale for $1.99. If you've not yet tried this series, they're at an excellent price to try them out. Don't be fooled by the $2.99 ones though, those are novellas.
All 4 have average Goodreads ratings of over 4 stars. So if you are in the market for a futuristic, tragic back story cop police procedural romance novel series, dig right in. I love these books because sometimes it's really enjoyable to stick with the same couple throughout a series, particularly without any irritating love triangles. And personally I don't figure that's a spoiler considering the 39th book in the series is about to be released, and it's always Eve and Roarke. That's right, 39th, so if you are at all interested in these characters, feel free to wallow, I know I do.