You would use 6P6 (Permutations) to find the answer.
You can also use the method that goes like this.
Book 1: 6 possible choices Book 2: 5 possible choices (as you cannot replace the book once you have placed it.) Book 3: 4 possible choices Book 4: 3 possible choices Book 5: 2 possible choices Book 6: 1 possible choice (as it is the last book that you have not used.)
