Two short stories that form prequels to the 'Red Queen' series; 'Queen Song', telling the story of Cal's mother, Queen Coriane, and 'Steel Scars', telling of Diana Farley's background. Although I imagine these could be read anywhere after 'Red Queen', I'm quite glad I read them after I had finished the series proper. 

'Queen Song' is quite a sweet little Cinderella story; Coriane is a minor member of an impoverished House, but nonetheless manages to catch the eye of Prince Tiberias. It gives  lovely insight into the relationship between her and her brother, and her best friend, which gives depth to the inferences made in the main series. Likewise, it makes you feel the gap that has been left in Cal's life, and even more hatred (if that's possible) for the woman who took her place. While there are no real plot spoilers in this prequel, if you were to read it before completing the main series I think some of the emotional resonance would be dampened, especially in the final book.

On the other hand, 'Steel Scars' does include a major plot spoiler for 'Red Queen' so under no circumstances should be read before you start the main series. I feel that this story wasn't as well executed; it's punctuated with coded messages between various members of the Scarlet Guard, but I found in most instances that they just got in the way of the narrative flow. I also don't think this tale does Farley justice; she's such a strong character in the main series, so I was hoping that this would really show her off. Unfortunately, it felt like her actions were limited to reconnaissance and you didn't get to see her in all her glory, in her own right. Nor did you get the background that might be expected. So this just felt like a bit of a damp squib.

Nice to have, but not necessary to enjoy the series.