The first book is called Wool and second is Shift. I've really enjoyed them so far.
It's about a post-apocalyptic society who live in a massive underground silo. The sheriff of the place elects to go "outside" which is essentially a death sentence as the outside it a toxic wasteland (or is it?). The story then follows a woman on the bottom floor (floor 144 (about a mile underground)) who runs the mechanical generator maintaining electricity for the whole "city." She's chosen as sheriff and uncovers a huge mystery surrounding the sheriff's death. Lots of interesting world building and mystery as you learn more about the silo. Highly recommend.
WikiwandSilo is a series of post-apocalyptic science fiction books by American writer Hugh Howey. The series started in 2011 with the short story "Wool", which was later published together with four sequel novellas as a novel with the same name. Along with Wool, the series consists of Shift, Dust, three...
https://www.wikiwand.com/en/Silo_%28series%29[https://www.wikiwand.com/en/Silo_(series)]