First Shift: Legacy (Wool #6) is the start to a prequel trilogy for five books of Wool by Hugh Howey. Without giving up too much of the storyline of Wool, the setting is a post-apocalyptic world where everyone lives in underground silos waiting for it to be safe to return to the above ground world.
First Shift tells both the story of how (and a little bit of why) the silos were built and the early years of living in the silos. The main character tells both stories through flashback.
If anything this is an even more engaging story than the early books of Wool. The book lengths is longer (over 200 pages) and the story feels more filled out. The time frame feels a bit off (the silos are built starting in 2049) but not too badly. I also think that the size and structure of the silos are not firm (they seem to expand and shrink through the books as needed.
But the books are still engaging and there is a running theme of this book about paternalism and doing what is good for the whole, even when it is bad for many.