What book is referenced in the equalizer?

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In the movie The Equalizer (2014), there is the following conversation:

Teri: What's your book about?

Robert McCall: It's about a guy who is a knight in shining armor, except he lives in a world where knights don't exist anymore.

Does anyone know what the book is that Denzel Washington (Robert McCall) is talking about?

Is it even about a real book or is this just a line for the movie?

Notes: Also in the movie, he makes a reference to Moby Dick which leads me to believe that this line is also about a real book.

What book was Denzel reading in The Equalizer?

What book was the movie The Equalizer based on?

Are there Equalizer books?

What is Robert McCall's background in The Equalizer?