Yes, an International Standard Book Number (ISBN) can contain both letters and numbers. The standard format for an ISBN is 13 digits, including both letters and numbers.
The ISBN of The Beatrice Letters is 0060586583.
The ISBN of Letters from Wolfie is 0670036943.
The ISBN of Letters from the Inside is 978-0440219514.
The ISBN of The Journal of Curious Letters is 9781590388310.
The ISBN of Letters to Ottla is 0-8052-3772-0.
The ISBN of Letters to a Young Brother is 978-1592402007.
The ISBN of Letters to Jenny is 0-8125-2282-6.
The ISBN of Letters to Milena is 0-8052-0885-2.
The ISBN of Letters to Felice is 0-8052-3500-0.
The ISBN of Letters from the Afterworld is 978-1-4575-0676-5.
The ISBN of Letters to Kurt is 978-1-61775-083-0.
The ISBN of The Screwtape Letters is 978-0-06-065293-7.