Unlike a lot of that late 30s, early 40s pulp sci-fi and fantasy fiction, Wellman's work holds up surprisingly well. Giants concerns a meteor crash that slowly starts to spread a red cancerous blight across the midwest, devouring everything and anyone that gets in its way. When two scientists discover that matter sampled from the core of the meteor has life-restoring properties, they set about using it to revive history's greatest scientists to enlist their help in saving the Earth. These includ Darwin, Edison, Newton, Marie Curie and Louie Pasteur. There are plenty of pulp-story twists and turns and melodrama, but all in all its a good story with a lot of interesting and clever (considering when it was published) scientific innovations.
Paired with Giants From Eternity is the shorter piece The Timeless Tomorrow, a chronicle of Nostradamus and how he deals with the politics of his time and how his life is affected by his gift for foresight, including his most dire predition: Atoma Divisia.
Though Manly Wade Wellman is known mostly for his later works (the stories of John aka Silver John) his considerable catalog of science fiction stands up just as well in my opinion.