I enjoyed this book...picked a brand-new copy up from the IMCPL Book Sale for $1.00. I would have liked it even better if there had still been copies for sale the next weekend. I only bought one so I could read it before deciding whether to buy some as gifts for my 4th grade students.
The book claims to be written by Ulysses Moore, but Baccalario's name does show up on the copyright page. There were a few things that annoyed me as I read - spots where some quick re-reading and revision would have made paragraphs much tighter. Also, as an adult reader, there were spots where the author over-defined certain words, i.e. labyrinth. However, as a teacher, I see the potential benefits of such redundancies.
I wouldn't use this book for a lit circle or read-aloud, but I'll certainly encourage some of my kids to read it. The characters have some interesting quirks and the mystery is suspenseful. Each chapter ends on a cliff-hanger, which would keep the more reluctant readers moving through the story.