Welcome to our forum.
I think the first thing you have to remember is that your husband may continue to say no, regardless of what medium of exposure you use with him. What will you do if that proves to be the case? Something to think about ahead of time.
Also, when he says no, is he saying it to *both*
of you doing non-monogamous stuff, or to himself doing non-monogamous stuff, or to you doing non-monogamous stuff?
"Do you think books, movies, or conversations are better? Do you join groups?"
I think your best bet would be to do a combination of all of the above; they work best in concert. Keep in mind, though, your husband said no to reading one book; he could say no to any number of things. That's strictly up to him. Perhaps there's some medium he'd be more open to (such as watching movies), but I don't know him well enough to say.
For polyamory in movies/music/art:
For book and website recommendations:
To find poly groups in your area:
Hope some of this helps.