Great idea, you've inspired me to do the same just now. I've been using PostgreSQL since ~2000 and love it, but I'm still finding new little corners of the docs worth reading.

If anyone else wants to do this and has Calibre installed, the commandline ebook-convert utility does a decent job. Change to the postgresql docs directory and then run

  ebook-convert html/index.html postgresql_docs.mobi --authors="Postgresql" --title="Postgresql 9.2 Documentation"

(or substitute postgresql_docs.epub). There are a lot of tweaks that'd make it better, but it's readable and properly handles the code font, etc. If you want much more than that using the Calibre gui might be a good idea. Biggest issue I'm seeing is that "Up" links & such are broken, so I'm working on that now.