adulatory biographies about his political career, lauding him for at least trying to get freed slaves and Indians a fair deal.
What happened, I think, is that northern liberals woke up and realized that while they had been ignoring the Civil War period, or focusing only on the experience of freedmen and Irish immigrants, southerners had been getting revenge by tarring northern heroes as murderous buffoons. Southerners portrayed Sherman's March to the Sea as an orgy of burning and slaughter, Grant's campaign against Lee as wanton butchery, and Grant's presidency as a disaster. None of this is true, and once they understood how greatly southern apologists had shaped the public view of these matters, and how connected this seemed to be to the new Republican politics, pro-Union historians went into action to defend their man.
For my view of Grant as a general, see here.