After building a book in publican the output is in a directory called 'tmp'. It might make more sense to name this directory 'output' as this is what it contains. From a developer perspective this may be a temporary directory, but from a user perspective this is the directory that contains the output of the publican build process. When I first started using publican a few years ago, I would routinely cd into the 'en-US' directory to find the output, as I had specified to build in en-US and this was the only directory I could see that seemed to relate. A directory called 'output' would be even more obvious.
It's not just output, it's full of intermediate content. If you want something clearer add --publish to the command line, it makes a directory name publish which contains only output.