What is the oldest Jacqueline Wilson book?

The oldest Jacqueline Wilson book is "The Story of Tracy Beaker". It was first published in 1991.

While Wilson had written other books before this, they were published under her married name, Jacqueline Willson. "The Story of Tracy Beaker" is the first book she published under the name Jacqueline Wilson, and it is considered to be her debut novel.