I'm working on one right now, with the help of a chap who knows about such things.
Interestingly (to me), I talked to my publicist about it and she advised me to make a short and general one for my series, rather than try to encapsulate the story of one particular book. Her sense was that I could use a one-minute trailer to show people the titles of the books, and give a vague idea of their genre and tone.
For me, to be honest, creating a trailer is a positive action I can do during a relatively quiet time in my publicity schedule. I hate sitting here, wishing I could do more, so this is something I can do that may not help but, unless I make a total c__-up of it, it shouldn't hurt.