Enigma by Graeme Base

Don't worry,' Bertie said at once. 'I'll solve this, have no fear, For magic things that vanish almost always reappear . . .' Bertie loves to visit the Retirement Home for Elderly Magicians and watch his grandpa perform magic tricks. But one day all the magicians' props go missing, so Bertie sets off to investigate . . . Can he solve the mystery in time to save the show? You can help Bertie find all the missing things and reveal the mysteries of Enigma using the special code at the back of the book. Good luck!