@pallazzio You don't have to! The Downside to Perfection is written so that you don't have to read the other one (which is All The Things We Lost). The two books only have one character in common (Jason), and The Downside to Perfection follows his journey. It's really a separate story, playing out a few months after the first book, with a completely different storyline.
However, if you wanted to read All The Things We Lost too, you should read that one first -- The Downside to Perfection might have some spoilers about the ending of that book.