Is it possible to like a book and not really like the characters? I picked this up because it's the "it" book of the moment and the comparison talk to Gone Girl intrigued me. It's not the same story, but employs similar elements - a multi-person 1st POV, a suspenseful layer peel to the last few pages.
For the tl;dr: Rachel is an unemployed alcoholic, obsessed with her ex-husband and her former neighbors. She passes both houses every day on the train and soaks up as much as a voyeur can. When the neighbor-wife disappears, she inserts herself into the ensuing investigation.
Megan is the neighbor-wife, emotionally bruised and bored. You probably don't like her immediately, but the gradual reveal of her secrets may inspire sympathy.
Anna is the current wife of Rachel's ex, entitled and spoiled but not entirely deserving of her fate.
I can see this as a movie. I'm sure it's been optioned. Worth a read if you can get through the characters.