This is an unsettling book.
Ripley is a non-entity, whiny, unimpressive. A nobody, as Marge very insightfully observes in a letter. Which makes him eerie, and by all rights not a character we should wish to root for. Yet from the middle on, I found myself anxious over the instability of his position. That's some writing for you.
The other way the book is brilliant is the subtle, but steadily rising, feeling that Tom is... not right. Even from page one there is this undefinable wrongness. Then there is some point around a third in where all the exclamation progress-posts start, and I totally got it when I reached it. That's one scary, sick puppy.
Think about it: he's a fast stepper, but he's no genius, and he likes to take chances; but he's a cool cucumber, and can mimic to convince even himself. No one realizes. Translate it to the real world now.
It is the horror of the uncanny valley, made all the scarier because we understated the only reason we know it is that we are reading from the inside of his head. Cheery though for before bed, huh?