A lot of writers I know have particular songs they associate with their projects. Famous authors such as Rainbow Rowell keep playlists for books they write and then share them with their fans. I think this is a really cool idea: sharing your book music with others. It helps both the writer and the reader envision the story better.
So, what sort of music should you be looking for? That’s entirely up to you. For me, when I hear a certain song and the lyrics and/or tone fit perfectly with a character, or theme, or even just one particular scene, my brain floods with images of my story. It even pumps me up for writing and editing later on.
If anyone is interested, I compiled my Timekeeper soundtrack here: https://play.spotify.com/user/tssim53/playlist/2kQ7JKXm7OIg0U03QqkHD9
What sort of music best fits your book?