The Testaments by Margaret Atwood is the sequel to The Handmaid's Tale. The book is great. It, too, will probably be in my top three for 2019 (along with Olive, Again).
The Testaments takes place fifteen years after The Handmaid's Tale. The story is told in chapters that vary from being written by Aunt Lydia and testaments written by two women, one from Gilead and one from Canada. Aunt Lydia's chapters tell the story of her life and her accumulation of power in Gilead. (You will be surprised to hear of her life before Gilead!)
The book is almost like a spy novel in that someone was a mole in Gilead. And the order in Gilead began breaking apart.
It's a rather big book, but I read it in just over two days. It's that fascinating! I would certainly recommend that one read the Handmaid's Tale first (it's a great read, too).