Jane Jacobs of Porter Square Books recently saw a customer point to “Between Shades of Gray’’ and refer to it as the “it’’ book of the moment, the S&M romance novel by E.L. James that was just released by Vintage as a paperback with film rights already sold for $5 million. Jacobs quickly stepped in to correct the customer and explained that “Between Shades of Gray’’ is actually a young adult novel by Ruta Sepetys about a 15-year-old Lithuanian girl who, under Stalin’s orders, is deported to a work camp picking beets in Siberia. The steamy James novel is actually called “Fifty Shades of Grey,’’ Jacobs explained. The books should not be confused - although because of their titles they might be. That’s why Porter Square Books sent the following tweet on Tuesday, just a few hours before Sepetys appeared at the Cambridge bookshop to read from and sign copies of her “Between Shades of Gray’’: “Dear Internet Please do not confuse the release of 50 Shades of Grey w/our event tonight for Between Shades of Gray.’’ The tweet was preceded by one that said “Are you missing something in your life? 50 Shades of Grey has arrived in the store! Call & reserve your copy now!’’ Confusing, right? Jacobs told us Tuesday that she hoped fans of the erotic novel wouldn’t accidentally show up to the “Gray’’ reading only to hear a story about a teenager’s harrowing experience in Siberia. “I have no idea what’s going to happen,’’ Jacobs said with a laugh.