Ask HN: Portable Shell Scripting resources?

Most 'Shell Scripting' resources seem to focus on Bash, which is a nice shell... but it has it's own nice set of incompatibilities with other shells.

I'm looking for a good book (or resources) that either doesn't include bashisms, or even better discusses ways of working around them.

I'd prefer if it's pretty comprehensive. Something that's hopefully gonna last a few years, I don't need a beginners text.

obviously if it teaches POSIX that'd be good, but not strictly required. As I was saying last night, I don't care if it's POSIX or not if all the shells support it (or at least bourne shells).