Fairest - Marissa Meyer Fairest - Marissa Meyer
Most of the time, when I see a .5 in a series, it's a 30-70 page tale to fill in a narrative gap, or keep the fan base appeased, or something that doesn't come across as unintentionally offensive. Leave it to Meyer to toss in a 200 page .5 instead.  
So ever wonder what turned The Lunar Chronicles Queen into the evilest royal this side of the Brothers Grimm?  Yeah, me either. But, Meyer answers the unasked question.  From her horrible childhood to her mortifying adolescence to her even worse adulthood (aside from ruling the Moon), this book takes us through the Levana's development -- descent? -- while planting all the seeds we need to see how the events of Cinder etc. are set in motion.
At the end of the day, I didn't need this book.  Sure, it humanized Levana a bit, but the Chronicles doesn't demand that (well, I might be wrong on that front -- I might revise this after I read Winter).  But, it was a quick read.  It was  interesting enough to see the wheres, whys, and hows behind the Moon's approach to Earth.  It kept my interest, and more than once, I forgot I was reading a prequel.  That's a plus in my book.
Source: http://irresponsiblereader.com/2015/04/28/fairest-by-marissa-meyer