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 :)