The notation that "it is primarily a preservative" cited by the original poster suggest that citric acid is the right choice. Lemon-flavored salt isn't a preservative. Also, I would not rely on the indexer for determining this, since the actual recipe and notes are not available that way. The fact that the indexer doesn't mention lemon salt in connection with the recipe - although the original poster did find such a reference - indicates that the indexer is not returning all of the information in the recipe.
I looked at the entry the "Pantry" section of the book which happens to be viewable on Amazon: " . . . this 'sour salt' adds a concentrated lemony flavor without adding the additional juice. . . ." That leads me to believe that when the author wrote "lemon salt", she was referring to citric acid, and the use of the term instead of "citric acid" is an editing anomaly.