I’ve done a fair amount of research on Grant’s drinking. Several things became fairly clear.
Grant throughout his life had a real problem with alcohol. Not with constant drinking, but stopping once he got started. IOW, he was a binge drinker. He tended to drink when he was bored or unhappy, especially when away from his wife, who he adored.
His enemies used his past history of drinking as a cudgel against him during the war. Lincoln specifically sent a semi-undercover operative, Charles Dana, out to hang with Grant’s army to find out the facts. Dana said there was no truth to it and became a big fan of Grant.
There is no solid evidence of Grant ever being incapable or impaired by alcohol during a battle or other important event.
Don’t know anything about the book specifically, but any book with that title is by definition looking for controversial stuff to include.
If you’re going to use this as a quote from Grant, imo you should have a contemporary source, not just a recently published book.
I’ve read other things Grant did say about slavery, and basically it was that he didn’t care one way or the other.
FWIW, before the war, when he was in dire financial straits, Grant freed the only slave he ever personally owned. To put this into some perspective, freeing him rather than selling him was the rough equivalent of giving up a year or more of income.
In fairness, the TV show did say that Grant was drinking more because he missed his wife and young child.