If the Buddha Dated
This is a book I've found helpful, read several times over the last 7 years, and shared with with a lot of people I've cared about. It is not actively about poly at all (and indeed, does seem to be geared towards finding "the one", but I've found it good for dealing with dating "the ones"), but is at least not slanted towards heterosexual relationships. It has helped remind me of the importance of being authentic and true to myself when forming new relationships.
I suppose I'll say a bit about why I think this is a useful book. For me, I think this is great for those of us who have occasions of worry or insecurity (those ones stemming from "no-good-reason") within a relationship, and I often open to anywhere randomly and find some perspective that calms me right down.
Luckily I haven't had much of this other stuff in my life, but it seems to have lots of helpful reflections I'd take for people who have trouble finding healthy relationships or are finding themselves unsure if a partner is a good fit or not. It's got good advice on how to figure out where to draw personal boundaries for yourself, if you find you have conflicted feelings about defining them with partners/etc.