While I am not an expert on baseball fiction, I loved Harris's novels about Henry "Arthur" Wiggen. Harris knew that baseball players, with the exception of his fictional catcher Red Traphagen (a college professor after his retirement), were not intellectuals and Henry Wiggen wrote, especially in The Southpaw, like a rookie pitcher not far removed from high school. His language was not precise and his grammar wasn't perfect, but he sounded like a 1950s kid.
Bang the Drum Slowly is the most book of the series as it was made first into a live TV production for CBS in 1956 with Paul Newman as Wiggen and Albert Salmi playing Bruce Pearson, the slow-witted catcher dying of Hodgkin's disease. There was also a film released in 1973 starring Michael Moriarty and Robert DeNiro.
But I prefer The Southpaw, which introduces Henry Wiggen as a high school star. You meet his father, a skilled Sunday pitcher in his town. His girlfriend, and eventual wife, Katie (whose father is an astronomer), and the rest of the New York Mammoths. Besides catchers Traphagen and Pearson, there is manager Dutch Schnell, coach Joe Jaros, and Henry's boyhood idol, pitcher Sad Sam Yale, whose first words to him are "Go f--- yourself." Henry quickly learns that baseball is a business and often a cruel one. The novel also touches on interracial relations (the book was written in 1953), the Korean War, and the role of the media in covering baseball.
The Southpaw is not all that easy to find in libraries now, but it is well worth the hunt.