I'm reading "Multipule Personality Disorder from the Inside Out" right now.
MPD being DID's old name. If you want to see you're not alone, or crazy, it's good.
It's written by people who have DID/MPD. It's kind of a collection. I don't like the poetry much, but that's just me.
I have read the DID Sourcebook. It's good and full of information, but a bit cold. The contacts directory in the back can be valuable.
It depends on what you're looking for.
But as far as people not believing? There's as many reasons as their are people.
Some can't admit there's the kinds of abuse towards children, or the volume of abuse out there in the world, to cause such a thing. Some can't accept it because doing so means accepting something else. That they were abused. That they knew about it. That they were blind to it. That they are guilty of it. Maybe it's not an endless list, but it's pretty long.
Accepting having it myself is hard. for more reasons than I want to type out.

Jax