The Mystery of King Karfu by Doug Cushman, 1996.

Seymour Sleuth, an Australian wombat living in London, introduces himself as “the greatest detective in the world.”  His friend, Abbott Muggs, a mouse, is a photographer who assists him in his cases and documents them.  When the story begins, Seymour receives a telegram from his friend Professor Slagbottom, who is working on an archaeological site in Egypt.  Someone has stolen one of their finds, the Stone Chicken of King Karfu, and he needs Seymour’s help to find it!  Seymour and Muggs head for Egypt!

King Karfu was a wealthy pharaoh and a wonderful cook, and the Stone Chicken may provide clues about the Lost Treasure of King Karfu, the nature of which is unknown.  When they reach the dig in Egypt, Professor Slagbottom explains that he was researching a message in code on the outside of the Chicken when it was stolen.  The suspects are the other people on the dig, who may be trying to steal King Karfu’s Treasure.

Seymour interviews the suspects one at a time and considers their connection to clues found at the scene of the crime.  As an adult, I figured out who the culprit was pretty quickly, but for the benefit of child readers, Seymour provides notes about the clues and suspects to help them understand the connections. The pictures in the story also provide important clues.  After Professor Slagbottom’s decoder is stolen, Seymour realizes who the thief is.

After they get the Stone Chicken back, readers can use the decoder provided to solve the code and learn where the Treasure is.  It turns out that the Treasure is actually a recipe, written in the same substitution code – for peanut butter and jelly sandwiches!

I always like mysteries that involve codes and puzzles, and this cute animal mystery would be fun and challenging for young kids.  With the key provided, it would be a good introduction to substitution codes for kids who have never seen them.  There is one other book with Seymour Sleuth, The Mystery of the Monkey’s Maze.  The author, Doug Cushman, is also the author of the Aunt Eater Mysteries.

The book is currently available online through Internet Archive.