The Selection Stories: The Prince & The Guard by Kiera Cass
After finishing all three books in The Selection series, I checked this out of the library, mostly out of curiosity. The first part is from Prince Maxon's perspective starting one week before the Selection begins and ending on the first day of the competition. And the second part is from Aspen's perspective as a guard inside the palace walls. Needless to say, it was a super quick read and a nice supplement to the series. I think, technically, it's supposed to be read before The One, but I read it after and it was fine.