The story starts off with a very strong premise: A former patient, Tanya Bigelow, wants his help in contacting Milo to get some help in checking out what seems to be a death-bed confession from her adopted mother, Patty, a highly regarded ER nurse. Alex correctly points out that there may be no crime at all, but Tanya wants to know. Tanya becomes Alex's patient again as her obsessive-compulsive tendencies resurface in the wake of becoming an orphan for the second time. Part of the appeal of the story is that you'll be rooting for Tanya throughout. That makes it easier to emotionally relate to the mystery.
The book soon bogs down into an extensive procedural that involves checking out all of the crimes associated with the areas around the places where Patty and Tanya had lived. Very little of this digging turns out to be relevant to solving the Patty mystery . . . but new questions arise as the investigation triggers strong reactions that pull Alex and Milo toward different puzzles. Along the way, you'll meet some pretty unappealing (at best) and disgusting (at worst) characters who will usually surprise you by acting more unpleasantly than you expect. I felt like I needed a long shower with lots of soap to get the filth off me after reading this book.
The characters are either drawn in very simple terms, or stay exactly the same throughout the book, with the exception of Tanya. Thank God for that! Otherwise, you will often feel like you are reading an overly plot-driven novel where the characters don't matter.
If you like intricate plots, you'll feel like you've got a dandy here. For my taste, the plot development seemed like it was overdone. I would have been happier with a simpler plot that moved along faster in a shorter book.
If abnormal psychology is your thing, Obsession is rich in that vein. You'll get keen insights into at least two different types that you probably don't run across every day.
The book's charm is mostly wrapped up in many short sequences involving Blanche, Alex's dog. These sections are fine enough to make you want to see a whole book that revolves around Alex and Blanche.