For class I'm writing a paper about the historiographical shift in respect to U. S. Grant's handling of the Civil War, but I'm having trouble finding sources from the 1950s/60s that view Grant negatively. Does anyone have any suggestions? I've tried biographies of Grant from the time period but they focus mainly on his presidency. I've also tried several biographies of Confederate generals and found some decent information there. I feel as though I'm not looking in the right places, though, and would appreciate some help.
That's a tough one, as most of my knowledge on the subject comes from fairly well-regarded histories. For example, Bruce Catton wrote two Grant histories, 'Grant Moves South' (1960) & 'Grant Takes Command' (1969) not long after the period you mentioned. I would say that these are two very interesting, well-researched books that give a fair accounting of U.S. Grant as a general during the ACW, even after approx. 50 years of new research (so not especially good for your purposes).
If you're looking for a more southern-slanted perspective, you might try Douglas Southall Freeman's 'Lee's Lieutenants' series, with a particular eye towards vol. 3. If you're going to go that route, bring a machete and rations, however, because that is some dense, THICK reading. That 3rd volume was published in 1944, I believe, so again, it isn't precisely in the time-frame you mentioned, but it is close.
All that said, I'm realizing now that this probably isn't much help in terms of tracking a shift in opinion on Grant through 50-60 years of 20th century historiography. If you had the time to do it, reading Freeman's 3rd volume, Catton's 2-part series (which is actually a breeze), and maybe a more modern piece like Jay Winik's 'April, 1865' might give you something to connect the dots. By that I mean with your proposed thesis in mind, maybe reading these books will help you uncover a pattern that you can identify? Anyway, I hope that helps!