Never. When he died is when he stopped. Over 7,000 pages still exist today.
in mirror writing
Leonardo da Vinci wrote most of his personal notes in mirror writing. Most of his notebooks were written in mirror writing.
That he wrote backwards ("mirror writing").
... that he used mirror writing.
Yes
About 1587.
In his notebooks
He wrote a treaty on Painting but, Leonardo's writing that is famous wasn't so much a book. His "Notebooks" are his most famous writing.
His great imagination and curiosity.
Some of his notebooks are in the Museum of Unnatural Mysteries.
Paintings and inventions.
His notebooks aren't necessarily a masterpiece (of literature). Leonardo's notebooks were actual notebooks, meaning that is where he sketched stuff, wrote down his ideas, drew his designs and even made notes about his personal finances. Leonardo's notebooks are a candid record into his life.