When this other appears intentionally in a work of fiction, it's called the alter ego. Philip Roth introduced Nathan Zuckerman as an alter ego, and over time, Zuckerman became Roth's double. The Zuckerman Novels are now part of university curricula worldwide and include two of Roth's masterpieces.
Operation Shylock, published in 1993, has a complex structure and plot that challenges the reader. The prose is of high quality and rhythmic drive. The novel explores themes such as duplicity, truth and lies, and the place of fiction in a clashing world. The title refers to Roth's mission in Athens for the Mossad, and the story ends with a meeting between Roth and Smilesburger in Manhattan.
Operation Shylock holds an important place in Roth's body of work, coming after a nervous breakdown he experienced. The novel won the PEN/Faulkner Award for best novel in 1993. In my opinion, while the plot is ambitious and at times lacks direction, it is still a remarkable work by a great writer.