Timewise, The Prince overlaps the beginning of The Selection and provides a glimpse in the Mind of Maxon as his father rounds up ladies for selecting. I'm not a Maxon fangirl, but I am a fan of reading all parts of a series when possible. Currently, this novella is available as an ebook, but my library shows it will be released as a paperback (along with The Guard) a little later this year. 
 In my opinion, this series is a good way to get angry with men/patriarchy and riled up over women's rights...but I'm not always sure that's the author's intentions. And the reader reactions are bit more swoony than I'd expect.