In the context of Spielberg movies, the Grail diary is the book kept by Sean Connery’s Henry Jones Sr. to keep track of all of his information on the Holy Grail.
So while there aren’t many overt references to Steven Spielberg movies in Ready Player One, the Grail diary from the book almost played a massive role in the film.
In the book, it’s what Wade Watts uses to keep track of his information regarding James Halliday’s Easter egg and plot his strategy to win the contest.
It likely would’ve been easy to include in the film, but Spielberg ultimately decided to keep it out of his adaptation, which has me wondering if that was a way of getting back at Ernest Cline for his digs at Indiana Jones in the book.