The first half of the book is good.
Carroll makes a point that he wanted to start the book with a jolt, but was ultimately argued out of it. He also points out that this is one of only two realistic books he's written - not much in the way of talking dogs (though there is a bulldog); not even any fountain pens. Those two items combined may explain the result.
The book is made up of three parts, and the first two (the most realistic) are good. Well written, interesting, smooth - I was looking forward to writing a positive review. Yes, there were some weaknesses (e.g., like all of Carroll's narrators, this one is highly judgmental, and often wrongly so, by my lights). But largely, the story moved forward nicely.
In the third part, everything comes apart - for the protagonist, but also for the story. Actually, that happens at the very end of part two, when he makes a difficult decision, but Carroll somehow mixes in some very disturbing sex 'play'.
In the third section proper, the characterization goes off the rails. While to some extent, Carroll is depicting normal teenage alienation, the extent of it here was difficult to buy, and is more papered over than explained. More disturbing, though, is that a major plot point turns out to be something of a red herring, deliberately concealed until the point where we find out and feel cheated. It's a major flaw of construction, and for no really good reason except authorial convenience.
At the end of the book, there's a stab at introducing some magic a la Carroll, but then it's withdrawn. I wasn't sure what the point of it all was - it's not as if Carroll couldn't have gotten away with magic. In fact, it's what readers expect. Instead, we get a strange hybrid that doesn't work well in either category. I wonder here whether there was too much or too little editorial intervention. The editor was wrong about the opening line. Maybe they should have focused less on that, and more on the other end of the book.
It's not the book I hoped it would be. The first half (two thirds, really) is strong, and enjoyable. If you're a Carroll fan, read it for that. For the rest,... it seems serious Carroll fans love it. If you're not one, you may want to pass.
On another front - this is part of the Answered Prayers series, to the point that it includes Finky Linky. I first thought this book should have been earlier in the series, but it turns out to be more of an alternate universe. It's not that Finky Linky.