“Sword and Scimitor: Fourteen Centuries of War Between Islam and the West” by Raymond Ibrahim.
Interesting that the post does not mention the name of the book, and also interesting that I cannot find the letter that Scott Perry wrote —
This one?
https://www.americanthinker.com/blog/2019/08/10_congressmen_hit_army_war_college_with_letter_decrying_surrender_to_cair.html