It was Bruce Catton's biographies of Grant (completing Lloyd Lewis's work begun in "Captain Sam Grant") that finally put to rest much of the rubbish published about Grant being "a drunk". Try picking up those or any modern biography (Joseph Edward Smith's "Grant" is recognized as the best one-volume study of the man published in modern times).
There is not one proven incident of Grant "drinking". It is widely believed that he did, indeed, resort to drink when he was posted to a remote California outpost at Fort Humboldt, and that due to loneliness after being separated from his wife and newborn son. The officers present with him at the time suggest the commanding officer (a pretentious martinet) was merely trying to make an example of Grant, and they themselves wrote in their own personal accounts that Grant was not a habitual drunk, was never unfit for duty, and, in fact, encouraged him to fight any charges the commanding officer was going to bring against him. Grant resigned, instead, because he wouldn't have any such charges brought against him, and he could not endure separation from his family any longer.
It was that one incident that started the military gossip that followed him his whole career, but, in fact, there is not one other recorded incident that stands up to scrutiny.
So I don't know what "history" books you're referring to but they obviously aren't the product of any kind of modern scholarship...and I certainly hope they're not Shelby Foote's books...he was a novelist and his books were based primarly on second hand sources...and are filled with inaccuracies. He repeats whole-cloth the story told by Sylvanus Cadwallader about Grant going on a "bender" during the Vicksburg Campaign, a story that is shredded by scholars.
So like I said, open a book, and not some weepy moonlight & magnolia tome written by a "Lost Cause" adherent.
Grant may have once engaged in drink, but beyond California there's no evidence he ever did. Did Grant drink? Almost certainly. As did every other officer in the army. I'm sure Grant had a drink here and there, and I don't see what bearing that has on the man at all. According to accounts Sherman could put away a quart of whisky a day.
All accounts about Grant are that he couldn't handle liquor at all. One drink would make him slur his words, two or three would make him "stupid". He had no tolerance for alcohol. And the accounts of various visitors to his camp, his subordinates, and those who encountered him throughout the war - in battle or not - speak to his gentle manner, quiet intelligence, "exceptional conversational" ability, and complete sobriety at all times.
If you knew anything about Grant you'd also know that he suffered intense migraine headaches throughout his life, induced by stress. He was, in fact, suffering one the night before April 9th, 1865. When his wife was with him, she would give him compresses for his head, and foot baths and such to help relieve his headaches. Those who saw this took this as signs of "hangover".
There is almost nothing to the rumors of Grant's drinking, and countless first hand stories and accounts that bear out this fact.