The original question and the update are two different situations. The example in the update is simply wrong. The object should always be "me". As was said, the rules get fuzzy in colloquial English and people usually ignore such rules violations. If you find yourself telling a story and you are unsure of which one to say, just say one and keep going, it's not worth stopping your story to figure out. However, if you are writing, it doesn't interrupt the story to fix it and you will be judged more critically, so do take the time to get it right.
We could simply pretend that the same answer (except "I" for the subject) applies to the original question too, but there is actually something else happening in the specific example given. When we ask the name of a book, we state the answer as "The name of the book is The Big Event" even though we ask, "What is the name of the book?" We switch the subject and object in the question and the answer. Same thing for the question, "Where is the book?" The answer is, "The book is there." "Who's book is it?" "It is my book." These questions CAN be answered in the same order as the question ("The Big Event is the name of the book." "There is the book." "The book is mine."), but it is not the standard. Actually, you would probably most often just get a simple answer, "The Big Event," "There," "Mine." The confusion doesn't exist for these simple answers because they don't change based on whether they are the subject or the object, but the common answers show that they are acting as the objects even though the question seems to put them as the subjects. Similarly for the question, "Who did it?" the most common full answer is "It was me." It is not incorrect to say, "I did it," but when we shorten it to one word, we are usually shortening the, "It was me," answer, so we just say, "Me."