Kat and the Missing Notebooks by Emma Bradford, 1999.

This book is part of the Stardust Classics series.

This time, Kat and her aunt, Jessie, find themselves in Florence during the Renaissance. They appear shortly before someone attempts to steal some very important papers belonging to the Florentine City Council from Signor Millori. Kat and Pietro, Signor Millori’s son, both run after the thief, and Kat helps to get the papers back. To thank Kat for her help, Signor Millori invites her and Jessie to stay with him and his family in their palazzo during their stay in Florence.

Signor Millori is a wealthy banker, and the palazzo is a grand place. Kat and Pietro become friends, and he confides in her how much he loves art. Pietro’s father wants him to become a banker, but Pietro’s dream is to study painting with Leonardo da Vinci. In fact, he takes Kat and her aunt to see Leonardo painting a mural at the Palazzo del Vecchio.

Leonardo notices Kat’s aunt and thinks that she has an interesting face. When he asks her to come to his studio to pose for some sketches, she eagerly accepts. Kat hopes that she can use the opportunity to show Leonardo Pietro’s artistic talent and convince him to take Pietro on as a student. However, although Leonardo likes the sketch of Pietro’s that he sees, he says that he has enough apprentices at the moment. Pietro cannot resist the opportunity to study Leonardo’s work, however, and borrows a couple of his notebooks to study.

Besides being a great artist, Leonardo da Vinci is also a scientist, and along with his notes on art, there are sketches and plans for possible inventions in the notebooks. Kat and Pietro go to return the notebooks the next day, but before they can get to Leonardo’s studio, the same thief who tried to steal Signor Millori’s papers steals the notebooks.

Florence has been at war with Pisa for some time, and the Florentine City Council has consulted with Leonardo da Vinci to see if he can come up with some inventions that would give them the upper hand. They know that the thief must be a spy for Pisa who is trying to see what Leonardo has been working on. However, Kat knows that if they do not retrieve the notebooks, history may be changed. Soon, it becomes obvious that the spy has an accomplice who is living in the Millori’s palazzo. Can Kat and Pietro find the notebooks and stop the spies before it is too late?

Although this is a fantasy story that involves time travel, it is based on historical events, and there is a section in the back that explains more about the Italian Renaissance and Leonardo da Vinci.  At the time of the story, Italy was not a united country.  The Italian peninsula was covered with many small city-states which frequently fought each other.  In 1494, Pisa broke away from Florence, and Florence fought for several years to bring it back under their control, until they eventually succeeded in 1509.  This conflict was the basis for the story, and the artist and inventor Leonardo da Vinci was consulted by the Florentine Council during the struggle.

Also in the back of the book is an explanation of mirror writing, which Leonardo da Vinci used in his notebooks.  There are also instructions for making fresco ornaments and a recipe for Nutty Biscotti.