Part A: On his most successful month, the author sells 70 books, and on his least successful month, he sells 30 books.

Part B: If we let x represent the number of books sold by the author in a month, the equation would be 30 ≤ x ≤ 70.

Part C: The minimum sales of books the author can typically have in a month is 30, and the maximum sales is 70.