There is another, helpful, free, alternative to Windows Movie Maker around the web, and it is Free Video Editor, a video editing software which has more features than the previous one and allows you to create professional book trailers. (advicesbooks.com)
You can see examples of professional book trailers created by the video tech experts at Self - Publishing Relief here. (selfpublishingrelief.com)
I chose these because they looked (to me) as though they could have been made without hiring a professional book trailer video company. (womensmemoirs.com)