Ernest Hemingway is/was the writer of the book...

Yes, I would say "was" in this case.

But it's a bit debatable in cases like "Shakespeare is England's most important playwright". He's dead, but he's still important.

Hi McTag
I agree. It's debatable. That's why I posted this query.

Once again you need to express what the context is of the sentence. Without the context in this instance, the answer can only be a debatable guess.

If you are talking primarily about the book, use 'is'. The convention is that the book exists in the present, therefore so does the identity of its writer. If you are talking about the writer and his life, it may be more appropriate to use "was". As always, context is everything.