I've listed them in order from shortest to longest:
A Persian Hero: Stories from the Shah Nameh by Wallace Gandy (120 pages). This same author also wrote a version of the Mahabharata: The Pandav Princes.
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The Story of Rustem and Other Persian Hero Tales by Elizabeth D. Renninger, illustrated by J. L. S. Williams (360 pages).
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The Shahnama of Firdausi done into English by Arthur George Warner and Edmond Warner. This is a multi-volume translation of the actual epic!
If I add any more books to this collection, you'll be able to find them with the Diigo tag: Shahnama.