I was asked to write an article helping people select music for their book trailer from booktrailer101.info, but I’m not going to put the cart before the horse. If I could wave a magic wand and make book trailers better, it would have nothing to do with music.

Books are read. That’s what they do. That’s their super power.
Videos are viewed. One does not read videos.

Get Voice-over on your trailers. You are allowed on-screen text only for the title of the book. That’s it. Everything else – Voice-over.

You can get really reasonable VO work for $5-$10 at fiverr.com. I’ve used them severall times for projects.
It is really easy.
It is really cheap

There is no reason to put text blocks in your video. Make it easy for people to understand your message. Go VO!

Now, did you get your VO for your trailer? Good. Let’s slot in some music.

Step one: Get a bunch of music. There are a lot of places you can get music for free. I maintain a list on my FAQ.

Get one or two pieces from each of those, or 10 pieces from one of them, but get different pieces. Get things you wouldn’t expect to use. Get a romantic piece, get an action piece, grab some weird march, try a piano concerto.

Step two: Try all of them.

Don’t get to one you like and stop… even if you think you found the perfect piece… keep going. Were you amazed at how effective one of the pieces made your trailer? Great! Use that one. Let’s not overthink. :-)

Step three: Mixing

Make sure you can clearly hear all of the voice work. If it gets difficult to understand in parts, turn down the entire music track. Make it easy for people to understand your message.