Depending on your definition of 'embedded', you might look at the various wireless router repackagings and scripts that are available..
Also, the NetBSD 'rump' kernel might be an interesting thing to review.
Generally speaking, the projects overall have excellent documentation, including manual pages or HTML docs on kernel, c library interfaces, building the system and packages from source, etc. Also, the whole system is in the source tree, which can be downloaded as a set.
Otherwise:
OpenBSD in general:
There's "Absolute OpenBSD', kind of more 'user/admin' level.
For lower-level stuff like API's, kernel organization, etc,
the McCusick books (Design and implementation of the {4.3BSD,4.4BSD, FreeBSD} Operating System), though either dated, or more specific to FreeBSD, respectively, still cover quite a bit of stuff that still applies to OpenBSD (most changes have been incremental, and can be tracked through the source tree history back to the original USG sources if needed)
I'd suggest running an install 'from source' on a spare machine/vm/etc for a while and reading docs and the source tree this will get you familiar enough with the system to have an idea where to go next.
Depending on your definition of 'embedded', you might look at the various wireless router repackagings and scripts that are available.. Also, the NetBSD 'rump' kernel might be an interesting thing to review.
Generally speaking, the projects overall have excellent documentation, including manual pages or HTML docs on kernel, c library interfaces, building the system and packages from source, etc. Also, the whole system is in the source tree, which can be downloaded as a set.
Otherwise:
OpenBSD in general: There's "Absolute OpenBSD', kind of more 'user/admin' level.
For lower-level stuff like API's, kernel organization, etc, the McCusick books (Design and implementation of the {4.3BSD,4.4BSD, FreeBSD} Operating System), though either dated, or more specific to FreeBSD, respectively, still cover quite a bit of stuff that still applies to OpenBSD (most changes have been incremental, and can be tracked through the source tree history back to the original USG sources if needed)
I'd suggest running an install 'from source' on a spare machine/vm/etc for a while and reading docs and the source tree this will get you familiar enough with the system to have an idea where to go next.