What book took Jane yolen the longest to write?

While Jane Yolen has written a vast collection of books, she has not publicly stated which one took the longest to write.

However, it's important to note that:

* Length of time is not always a reliable indicator of a book's difficulty: Some books might be lengthy but require less research or revision, while shorter books might demand extensive research or a challenging creative process.

* Yolen's writing process can vary: She has worked on projects across different genres, including poetry, children's fiction, and historical non-fiction. Each project likely presented unique challenges and took different amounts of time.

Ultimately, without a direct statement from Jane Yolen herself, it's impossible to determine definitively which book took the longest to write.