I thoroughly enjoyed both of these books. The Alienist is so well-researched and vivid you truly get lost in London and can almost smell Bedlam as Kreizler, the pioneer of mental health, helps to solve truly heinous crimes. The ecclectic group of characters (not the least of whom include Teddy Roosevelt) is a perfect set up for a classic mystery in Victorian London--with a serial killer kind of twist, of course.
The sequel is equally well-done, this time narrated by young Stevie. Carr does a phenomenal job showing how London's East End has forced him to be a man while still showing that he is, ultimately, a child. The bittersweet relationship he has with a young prostitute is a nice side story to main mystery plot.