My mom is fascinated by him and his connection to the "Rough Riders," Teddy Roosevelt, and WWI. I was looking into some good literature on him to get her for Christmas, but I don't know what is the best/most true narrative at this point. I know he has his writings on WWI in his own words, but I would love this subreddit to weigh in if you know anything particularly fascinating about this man I don't know much about either.
Yes, that's correct, there have only been two men ranked General of the Armies of the United States. John 'Blackjack' Pershing was also the only General of the Armies to achieve the rank in his lifetime - Washington was awarded it posthumously in 1976, though it had been proposed in 1799 to award Washington the rank. President Ford specified in Public Law 94-479 that Washington would outrank all Generals of the Armies both past and future.
General of the Armies signifies authority over all armed forces. The rank is not to be confused with General of the Army, currently the second highest U.S. army rank. General of the Army ranks above General within the structure of the army branch. It is equivalent with Fleet Admiral or General of the Air Force and is commonly today referred to as a "five star" general. Note, however, that while Pershing outranks a General of the Army, his insignia (which he was permitted to design himself) was the four stars of a General, but in gold.
The name of the rank General of the Army was revived after the Civil War era for use in WWII, to give American generals equal ranking to Field Marshall (used by other Allied nations) and Admiral of the Fleet. The Civil War era version could only be held by one officer at a time and was not comparable to the WWII version. Grant was appointed the rank in 1866 and wore four stars. Sherman was appointed the rank in 1869 when Grant became president and changed the insignia to two stars with the U.S. coat of arms between them. Sheridan was appointed to the rank in 1888 and it ceased to exist after his death shortly thereafter.
Pershing's appointment to General of the Armies was closer to the Civil War era version of the rank. It was awarded in 1919 for his leadership of the American Expeditionary Forces in WWI, so that Pershing would outrank the few other full Army generals of the time. Before his command of the Expeditionary Force, Pershing held the rank of Major General. He was promoted to full general when appointed command of the Force (which he held until 1918) and was the first of that rank since Sheridan. Then of course he was promoted to General of the Armies after the war.
Unfortunately, as to your questions in your explanation, I know relatively little about Pershing himself or his career. What little there is comes from readings on Patton, who served under him in WWI and took a strong liking to him.