Imagine three books as A, B and C, and list all the borrowing situations:

1. Borrowing books: A, B and C.

2. Borrow 2 books: AA, BB, CC, AB, AC, BC, 6 kinds.

3. Borrow three books: AAA, BBB, CCC, ABB, ACC, BCC, AAB, AAC, BBC, ABC, 10.

2×(3+6+ 10)+ 1=39