Step 1: Selection for one book
For any book, we can choose 0, 1, 2, ..., P copies. Total $P+1$ ways.
Step 2: Total Selections
Since there are $n$ different books, total ways $= (P+1) \times (P+1) \times \dots$ ($n$ times) $= (P+1)^n$.
Step 3: Non-empty Condition
Exclude the case where zero copies of all books are selected.
Step 4: Conclusion
Result $= (P+1)^n - 1$.
Final Answer: (A)