Hi, my name is Kim and I am addicted to books. See? I can admit it!!
Annabel is the story of Lena's mother and it really gives readers of the series a little bit of insight to her. It starts years before Lena was born and shares Annabel's experience with the cure and her time in the crypts. Annabel grew up as society was changing and she rebelled against it, eventually meeting the man who would become the love of her life, even if he would never be able to love her. It was interesting to get to know Annabel as more than a distant figure in Lena's life, although I wish it had been a bit longer so we could have learned more about her time following her escape from the crypts.
While you can definitely read the trilogy withou this, I think it offers some depth and understanding that makes the overall story even better.