The riddles. The motherloving riddles? How can you not be insulted by how stupid they are? It's the kind of riddles you get in a Christmas Cracker, only dumber. The kind of riddles that would momentarily confuse a newborn crack-kitten. For about a second.
I thought the riddles were probably a key feature of what made the book popular.
Like the old joke:
"When I'm with Umberto I feel that he's the cleverest person in the world."
"Why are you with Dan then?"
"Because, when I'm with him, I feel that I'm the cleverest person in the world."
The alternative, that people find the puzzles
I will, however, put this example in a spoiler on the offchance that someone does. I remember that most of the puzzles were about this difficult, but I have forgotten most of them.
How long does it take the characters to find out about mirror writing?
I'm just glad that I just saw the book in the library when it came out, so I didn't spend any money or much time on it.
Yes I did finish reading it because it wasn't bad enough that I actually stopped reading, just bad enough that I won't read any ore of his books*
*unless stuck somewhere with no other reading matter.