Is there any documentation available for creating new reports? I've looked in the Billings help and can't find any.

I'm trying to do fairly simple stuff -- e.g. I'd like a report showing me which clients/projects I worked on yesterday, at what times, and for how long.

+1 for this. Need more reporting docs specific to Billings!

There is this document, but it doesn't really cover the things I've tried to do with Billings reports:
Reporting Engine Concepts

Also, as far as I can tell, I don't see that the timers in Billings record a start and stop time: just the date and duration, so I don't think you could get a timesheet kind of report from it.

another vote for this... trying to create a report and struggling to find any help for this.