Tense, Aspect ~read / were reading / had read / had been reading~

In all of these sentences, "the same novel as" would never be said by a native English speaker. The phrase "a copy of" is optional, and you could also optionally insert the word "that" between "novel" and "you." Also, "couldn't" is more natural than "could not," especially in spoken English.

(C) and (D) are both extremely unnatural as you worded them. If you want to emphasize that the other person finished the process of reading the novel a few days ago, you can use my modified version of (C).

(B) is more likely than (A), because it generally takes longer than a day to read a novel, so it is strange to say that someone "read a novel" on a particular day. Even though "a few days ago" does not specify which day, it still suggests that the entire action of reading the novel happened at a particular time, rather than over a span of time. This is not a problem when the action is "finished reading the novel" rather than "read (past tense) the novel."

(B) is the only really natural sentence of the four, and only after the changes I made.

I'm very grateful to you for telling me the exact points I wanted to know.

Thanks a million, tonyさん!!