Since I still haven't tracked down "The Southpaw," the natural next choice in the baseball novel review series is "The Natural" by Bernard Malamud.
The common thread between all three baseball novels in the series so far is they've all been considered, in some capacity, the best baseball novel of all time. But that title is probably given to "The Natural" more than any of the others.
The difference between "The Natural" and some other baseball novels is that Malamud didn't write about baseball very often. In fact, "The Natural" was his first novel, but he never returned to baseball as a main topic after that.
Malamud may also be the only author in the series who's won a Pulitzer Prize. (Not for "The Natural" but for "The Fixer." I tried to read "The Fixer" once but I couldn't get into it.) He also won two National Book Awards.
One last note on "The Natural." This is another book that was made into a popular movie. In many small ways, and in one HUGE way, the movie is unfaithful to the novel. So when you're reading "The Natural," don't picture Robert Redford as main character Roy Hobbs; picture Babe Ruth.