(1) A how-we-did-it parenting book loaded with anecdotes from the authors' experiences and their friends, all of whom are writers living in New York (or, conceivably, some other dense city) in one bedroom apartments. While it would be okay to have a little wow our lifestyle changed when we had the first kid, that's covered adequately elsewhere. Highlights should include: how we (or our friends) juggle 2+ kids in a one bedroom apartment; how we respond to friends/family who try to convince us to move to New Jersey (or at least a two bedroom in Brooklyn), etc.
It's quite clear that the material is out there. I don't see any reason why they can't share with the rest of us. It'd be a nice supplement to the standard, reduce-your-footprint advice to live in a smaller home. Further, if it's written by a bunch of highly educated, high status but cash poor 30 somethings in New York, it _won't_ be all about elimination communication and the family bed. There will be an amazing amount of life contortion to adhere to middle class norms within a highly constrained space. I don't know about you, but I bet I'd learn a lot.
ETA: Also, the potential for huge snarking against Park Slope parenting makes me cackle with glee.
I would anticipate at least one chapter in the book being devoted to how the family carted the wee ones around on bikes at various ages and stages, and also a chunk devoted to agonizing over at what age will I let them ride the subway without adult supervision. Also, tech toys that don't work as advertised (GPS monitors, and similar).
(2) A how-we-did-it parenting book with anecdotes from the authors' experiences and their friends, who live in places like Seattle, Portland, San Francisco, Minneapolis, Somerville (MA) and other segments of Greater Boston, the Triangle in North Carolina and anywhere else with a family biking community. While it might or might not be possible to find enough families who went car free outside of dense city centers, it should be possible to find a ton of families who went car lite in city neighborhoods and inner suburbs (and a scattering out in rural areas). This entry can presume a relatively spacious family residence (2, 3 or even more bedrooms for a family of 3 or 4); the primary distinguishing feature would be that these families transportation choices don't include heavy reliance on cars.
My preferred form of non-fiction is I-am-curious-let's-go-find-out: the author stalks some experts, participates in some research, does a lot of reading and then sums it all up with the open questions in the field. About a decade ago, in the late 1990s, activist non-fiction could be summed up as blugeoning the reader with statistics, and following it up with some rah rah cheerleading about Things You Could Try to Make It Better. More recently, activist non-fiction has developed this I Followed A Bunch of Rules genre, in which the author does some batshit crazy thing with their loved ones tolerance and/or participation, mostly to get a book contract, some publicity, all in the service of Making A Point (usually along the lines of, wow, that advice is not as easy as it looks, it's depressing, families resist, and it would be a lot easier to accomplish related goals much more easily if we collectively decided to do X instead, altho occasionally, it's more about, well, it's still a pretty ideal and, even more occasionally, wow, look what a bad idea that lifestyle is. Virtually all of these conclusions being comparatively obvious from the beginning of the project.).
Parenting literature, by contrast, has a very personal, let's get our author friends together and write about what a harrowing experience ages X through Y were, as well as a fair amount of, my kid has Z, but my kid is way cool anyway. What I'm looking for is sort of a twisted melange of the two subgenres.
Because I think those books should exist, and actually, I could not write them.