I think I may be harder on non-fiction than fiction. If a topic's interesting, the author's done their research, and I'm learning something, that's a 3. In order to rise to 4-ness, it has to have something extra--be especially funny, or moving, or profound. (To hit a rare 5, it has to be a favourite, worthy of re-reading time and again--not necessarily a classic like "On the Origin of Species," but a 5 for me is something like Murder Ink or Walt Disney Imagineering, so there's no rhyme nor reason for what will particularly appeal).
This is a solid 3--for a novel, that would probably mean "you can do better if you want to," but for a book about walkable cities, it means "this one's fine." I found the author's thesis confusing (not sure she had one, per se--there seemed to be a lot of "this thing happened and the city's so walkable" but also "this completely opposite thing happened and this other city's so walkable," so that was a bit muddled.
The choice to zero in on a few neighbourhoods was also a bit puzzling (they certainly weren't the ones I'd have picked--instead of focussing on some outstandingly walkable places, she seemed to choose grudgingly walkable places).
(Note: 5 stars = amazing, wonderful, 4 = very good book, 3 = decent read, 2 = disappointing, 1 = awful, just awful. I'm fairly good at picking for myself so end up with a lot of 4s).