I distinctly remember buying this book in paperback at Barnes and Noble on a family shopping trip to New Jersey sometime in the early 1990s. I was drawn to the cover, but perplexed by the content, and even after I purchased the book and brought it home, I could never get into it enough to sit down and finish it. I believe my problem was probably related to the fact that this book invites its audience to help solve the mystery. I found this intimidating as a kid, and too gimmicky. I just wanted to be told a story. (This is why I also hadn't read The Westing Game until last year.)
As an adult, though, I found this book to be charmingly quirky, clever, and funny. I still didn't have any interest in solving the puzzle, but I no longer felt pressured to do so, and I appreciated the details that are included for the kids who do like complicated word games. I also loved the cast of characters, which is smaller than in The Westing Game, but still every bit as interesting and carefully crafted.
Another nice thing about Raskin's books is that they are so strange, they manage to transcend time. There is nothing in this book that I would consider "dated" because nothing in this book is really normal enough to recognize as part of real life at any time. For the right reader, this book will be as great a fit now as it would have been 40 years ago, which is probably part of the reason it was reprinted in 2011.
Highly recommended for Roald Dahl fans and middle school readers (and another great alternative to the very disappointing Escape from Mr. Lemoncello's Library.)