Something that has worked really well for librsvg - and now I'm happy to have for at-spi2-core as well - is a development guide for the internals. Not a library manual, but an internals guide.

It's available at gnome.pages.gitlab.gnome.org/a…

I want to make it a good reference for the implementation details of #accessibility - for how the system is built, the roadmap and the cleanups we're doing, how to make toolkits accessible, that sort of thing.