In the 1990s, a bunch of transit planners at Kittelson Associates got the idea of creating "Transit Capacity Manual" that could sit next to the ominous authority of the Highway Capacity Manual, and maybe talk back to it in its own language. The result was the Transit Capacity and Quality of Service Manual. It's quite a technical document, but it does endeavor to explain the main concepts of transit planning and especially its similarities and differences with highway planning. This is useful, because a lot of decisions about transit are made in local and state transportation departments where most people are trained as highway engineers.
The manual is now in its 2nd Edition, and the whole thing is online, here. Kittelson is now starting work on the 3rd Edition, and has set up a web survey asking for input from the profession about how the next version could be improved. If you've used the previous editions at all, or even if you just have strong feelings about what manual should do, you're encouraged to fill out the survey here. I just finished it. It mostly invites you to submit text comment, not just tick boxes, so it's obviously going to be read with some attention.