Install cask that extends the brew command :
brew install phinze/cask/brew-cask
Install calibre using cask :
brew cask install calibre
Setup $PATH for calibre (add ebook-convert and other commands to $PATH) :
export PATH="~/Applications/calibre.app/Contents/MacOS/:$PATH"
Or add it to your shell's .profile with :
echo 'export PATH=~/Applications/calibre.app/Contents/MacOS/:$PATH' >> ~/.profile
Now ebook-convert should be available :)
As of 2017, it looks like you no longer need to run
brew install phinze/cask/brew-cask.brew cask install calibreshould be enough to gain access to theebook-convertcommand on modern OSX.