The Prince Was an amazing Selection novella. I Think The best time to read it would be starting at chapter four of the Selection and then switching back and forth between the two as the story developed. It Introduced A new point of view with Maxon that shows he isn't necessarily as perfect as anyone thinks, and neither is his life. It Also Showed the King and Queen from his point of view, beyond the crown's and facade they present to the public. I'm Not Going to say much more for fear of spoilers, but you will love it and learn to love Maxon even more than you already do.
As The King never has his own book it is the novellas The Prince and The Queen that will provide clues to Himself from the point of view of his loving wife and adoring son.
That is so interesting that the king never gets his own story! It's cool though. Hopefully I can read it soon.