The Wright 3, a 2006 children's novel by Blue Balliett, a sequel to Chasing Vermeer.
Summary by Vijay Fafat from the Mathematical Fiction site:
This is the second mystery book with Calder, Petra and Tommy, where the events take place after those in "Chasing Vermeer". The main theme in the book is the impending destruction / tear-down of famous Frank Lloyd Wright house - "The Robie House" - on the University of Chicago campus. The art students of the university school are convinced this is akin to murder and try to find ways of saving the landmark. At the same time, some thieves are trying to rob the Robie House. And the kids hear that Wright had encoded his visage, his soul, into the house, which is a mystery they feel they must crack.
All ends well, with hints of some supernatural elements about a living house thrown in. As a bonus, they finally find Wright's hidden code in the form of a stick-figure, skeletal man drawn on a glass window, with body parts set in dimensions which form a Fibonacci sequence ("It’s a Fibonacci man! Wright coded himself in Fibonacci numbers!")
As in "Chasing Vermeer", the children communicate through a mathematical code, though the code used in this book is new ("Wright Sandwich Code", which uses pentominoes as a coding mechanism). Similarly, the book contains illustrations in which there are hidden fish-figures. It turns out that book chapters whose numbers are part of the Fibonacci sequence contain the same number of fish but illustrations in other chapters do not.