The Shift Omnibus continues with the underground silos introduced in Wool. However, this book is a prequel and takes place beofre the action in Wool. We begin to see and understand how and why the silos were built, although the answers take work to discover. We are introduced to the people who built the silos and then see their struggle to come to terms with what they have done to themselves and the world.
I really loved this book, but I thought it was quite different from Wool. Wool was fascinating and entertaining, always full of suspense and surprise. The Shift Omnibus I thought was a lot more thought-provoking, reflective, and very sad as the characters have to learn to live in this new changed world. Even the love stories in the book are quite unusual and will make you think and have a deep sadness to them. The whole book in general makes you think and possibly needs to be read more than once. I still loved it, but it definitely requires some thinking. Also, the timelines and characters shift so you have to pay close attention to the dates and the silo numbers or you will be very lost. I am glad I read this soon after the Wool Omnibus.
Starting in about Third Shift the action "caught up" and started overlapping with the one in Wool and the whole series took a very fascinating turn. I thought that was absolutely wonderful. You get to see the action both from the new characters and the old characters from Wool and of course get a bigger picture and better understanding of what is going on. If you liked Wool, this is definitely worth reading and in my opinion a great sequel or rather prequel, but it does have some important differences from Wool.