This book takes place between books 1 & 2 in the series, and that's definitely the order I'd recommend reading it, despite it being listed as 4th in the series. You won't lose anything if you don't, but I think it just flows better.

Really lovely little interlude though, and I enjoyed seeing the bridge between Idaho and Ethan coming to stay at the White House. Great depictions of the pressures on their relationship and some good build for the coming books.