The Boy Who Followed Ripley will either be your favorite Ripley book or it will be a large disappointment.
If you have not read any Ripley books, I suggest that you start with The Talented Mr. Ripley instead.
Those who will be disappointed by this book will be people who wanted a book just like one of the first three in the series. Those who will be very pleased are those who want to think through the implications of Ripley's character and who he is becoming. I have graded the book as an average of the two likely reactions.
We see a new side of Ripley in this book. He takes a troubled American teen under his wing and mentors him in the way that a friendly uncle or much older brother might. In the process, Ripley reveals more of himself to the boy than to anyone else. Ripley also ends up musing and seeing his own marriage and history in a new light as he understands the boy's problems.
I'm sorry that I cannot go into the story in more detail. To do so would simply spoil the plot development for you.
If you like character development with long stretches of little plot development, this book will be a lot of fun. If you crave the constant action of The Talented Mr. Ripley, this book will drag slowly in long sections for you.
Unless you are ambivalent about the Tom Ripley character, I do suggest that you read the book . . . even if it won't be your favorite.