Translation:This book was not started by them.
Are you sure? I think your sentence would be a more apt translation of "Buku ini belum dimulai oleh mereka". Those (with "has not" and "belum") seem imply that the book has not yet been started (and perhaps that when it is started we expect it to be by them). Whereas the sentences given here (with "was not" and "tidak") are consistent with the book having been started, but not by them (along with a number of other interpretations).