Second Shift: Order definitely has ‘middle book’ syndrome going for it, but in this case, it certainly works. You get a pile of answers to questions raised both in First Shift: Legacy and Wool along with filling out the world of the Silos.
It’s a fascinating dystopia and feels just real enough to be terrifyingly possible. I do prefer Order over Legacy in that it doesn’t have the ‘modern day’ / pre-Silo points of view. I find it more interesting to get hints of that world from after the fall, rather than having everything spelled out.