I adored Hana, Lena's best friend, and her vivaciousness in Delirium, so I was excited that she would be getting her own story. Hana takes place during the time of Delirium but now we're seeing the story through Hana's eyes. I was bummed that the fun-loving girl we knew from Delirium is gone, but considering the events that take place in Hana, it is understandable. The best part of Hana is the best part of any Lauren Oliver read, in my opinion: the writing, which was lovely and powerful, just like it always is. 
 Would I say that Hana is a must read? Well, no. I don't think it's critical in the overall series as for the most part, it is just a rehash of some of the events that took place in Delirium from a different point of view. However, there was one very significant reveal right at the end of Hana - a true OMG moment - but I can't say I liked it. (view spoiler)[I find it difficult to believe Hana would rat out Lena to the regulators for such an immature reason. I needed more development and explanation in this instance. I hope we will get this explanation in Pandemonium or Requiem. (hide spoiler)]