The romantic subplot is a big reason to stay in order. Noah and Josie's relationship has a very specific, slow-building trajectory with major setbacks and milestones.
If you read a book where they're married, then go back to when they're just partners, a lot of the tension and sweetness is lost. You'd spoil your own experience of their first kiss, their first 'I love you,' their arguments and reconciliations. Their love story is woven through the crimes, and it's worth experiencing in real time.