There isn't much out there. If DP is really OK extrapolating from a lesbian identified non-bio-parent experience, then yes the Confessions of the Other Mother anthology edited by Harlyn Aizley is pretty good. Several of the essay authors identify as some level of genderqueer, so there might be some things that are directly helpful as well. I feel the need to put a bit of a caveat on the rec though, because I actually came away from that book completely petrified. There is definitely a not-so-subtle message in many of the essays that if you didn't give birth, and you are not nursing, you are less of a parent than your partner (I could pull out plenty of quotes, but will refrain here). Your DP may be less sensitive to that though if you guys are not having to grapple with how to share the "mom turf." (Thankfully, my fears turned out to be unfounded in my own family)
I also second the Brill recommendation. Compared to, say, Rachel Pepper's book (which really misses the boat on non-bio parent issues), she really does nail a lot of what comes up for non-bio-parents, and yes, the focus is on lesbian non-bio-moms, but is fairly inclusive in terms of language. DP can just skip all the pregnancy/TTC stuff and look for the parts where they talk about relationship issues that may come up.
I also had some luck with handbooks for Stay at home dads, interestingly. They tend to assume competence, which many of the "dad" focused books do not. Can't remember specifics, I just know I tended to get them out of the library before our daughter was born.
Good Luck,
Lyn