When the group falls into the library in the Neitherlands, the librarian calls Margo "Janet" before she corrects her. This is a reference to her character's real name in the books (that was changed to reduce confusion between her, Julia and Jane).
Marina, Kady, and Pete do not exist in the book series. However, the nickname Kady uses with the Free Traders, Asmodeus, does exist in the books.
Hale Appleman (Eliot) has said in several interviews that he first auditioned for the role of Penny and Arjun Gupta (Penny) originally auditioned for the role of Eliot before they each received the other role. In a March 2016 interview with The Observer, Hale Appleman said, "My best friend gave me the books when he heard I was auditioning for Penny. The one-line blurb about Penny was that he was punkish, but with a Clockwork-Orange-like attitude, which is something I feel like I could connect to, but that's not the whole picture of the character." The best friend was fellow actor, Anthony Carrigan.
The first season is based on books written by Lev Grossman. Quentin's story is based on the first book in the series, The Magicians, and Julia's story is based on part of the second book, The Magician King.
Based on a trilogy by Lev Grossman: The Magicians, The Magician King, The Magician's Land.