A full outer join preserves every row from both physical tables, returning matched records where possible and null values where a corresponding row does not exist. That directly satisfies the requirement to retain books that appear in only one of the two incomplete tables. A Tableau relationship can also satisfy the requirement because relationships keep logical tables separate and defer physical joining until analysis time. Tableau dynamically determines the appropriate joins based on fields used in the visualization and makes the row and column data from related tables potentially available without permanently collapsing the tables. Left or right joins would discard unmatched rows from one side, while an inner join retains only matches. Therefore, Full outer join or Relationship model is the correct combination.