Update Note : I'm in the middle of book 3 at the moment and can finally confirm that it is not completely necessary to have read this short story before reading book 3. However, there is a huge tie-in of one particular instance which happens in Radiant with something that will take place in Boundless. The situation is explained enough in book 3 to where you can get by if you skip this story, but I liked knowing what happened in Radiant first. 
 Angel books are probably even worse than vampire books for having to slog through a whole lot of mediocre in order to find the gems. 
 Truth be told, I tend to prefer the Angelfall style of angel lore... where the vibe is dark and the angels are suspect. I have a harder time with the manufactured sappy-romance type of angel stories. 
 Thankfully, the Unearthly series is pretty decent for an angel set of books which has more romance than horror. I gave high marks for the first two books and am anxiously awaiting the third. 
 Radiant is a short story which takes place between books 2 and 3 and the majority of it revolves around Angela. This particular story was just okay - interesting enough, but nothing to find myself excited over. I'm not even sure how necessary it was to the series. Other than getting to find out about an angel living in Italy who drives Angela's passion, we didn't really learn anything else new about information having to do with Clara's story. 
 All I can think is that this angel will probably show up in future books to help drive the story along, so I guess if you feel the need to know about this character before he's brought back later (even though I'm sure he'll be explained if/when he makes a reappearance), go ahead and read this. It's such a short story that it will only take but a few minutes to read, so you won't be wasting your time even if you don't find out much in the way of new information. 
 Regardless of how underwhelmed I felt about this side story, the series itself is absolutely worth the read if you enjoy stories of love, destiny, and the journey of one girl to find out who she really is.