When you are writing a book in a different era, or even one in current times, music can help you get into the mood of the book. For example: if you are writing about the Roaring Twenties, play jazz or ragtime to put you into the mood and mind set. If your book is set in Greece, put on some Greek folk music - anything that suits your story. Music can remind you of experiences you have had and evoke emotions that would otherwise be dormant in your subconscious. In addition to all your other research, music is a great tool.