It's this one. Here's a taste of it. I found it fascinating because the book was written specifically for transport planners, providing a concrete methodology for prioritizing transport measures based on principles of justice and equality. Basically, the book felt like it was written for me. If all goes well, I'll be Skyping with the author next week to get some input on my ideas.
The gist of or is this: Transportation planning is mostly about government bodies deciding where to allocate their resources based on the locations that they give the most priority. Setting priorities and distributing resources are inherently normative activities. Currently, planners prioritize and distribute resources based on their expectations of congestion and on cost-benefit analysis. Both of those methods are disproportionally favoured towards the non-poor. The book argues that a more fair transportation planning method should focus on population groups that suffer from the largest accessibility deficiencies.