‘Harriet Beecher Stowe’s great novel, Uncle Tom’s Cabin, had been an enormously popular serial novel in the abolitionist periodical The National Era.’ The question said that we need to delete the commas to make the title of the book a essential information. What if that was a phrase or a clause? Can we still delete the commas and make it essential clause?
What if WHAT was a phrase or a clause? Are you asking about a sentence fragment? I don’t understand your question. Uncle Tom’s Cabin is essential to understanding this sentence. Therefore, there should be no commas around UTC. However, if the previous or next sentence uses the name of the book, then maybe in CONTEXT, the commas would be needed. Except you probably wouldn’t be saying Uncle Tom’s Cabin in this sentence at all, if that was the case.