Rabbi Joshua is a Jewish djinn of the Jinn tribe that Nimrod met in the sixth book. He is a Kabbalist, which means he believes that every number contains a secret meaning. His room in The Hebrew Library is covered with the tree of life, pictures of famous mathmeticians, and the Temple of Solomon.
He is a librarian The Hebrew Library, which contains a vast collection of things, including Einstein's original papers. When Nimrod finds out about Shamba-La he goes to visit him to borrow a books about the subject. However, because Rabbi Joshua is a very particular man he would only give him the books if he would gamble three wishes for them.
Afterwards, it is found that Rabbi Joshua's Einstein papers, as previously mentioned, were stolen by Jirjis Ibn Rajmus.