Code blocks are cool but can be cooler. This plugin adds lines numbers for multi-line blocks and a copy button to easily copy the content of your block.

The next image shows a single line code block:

When displaying code with multiple lines, line numbers will be added:

You only have to edit your book.json and modify it adding something like this:

"plugins" : [ "code" ],

This will set up everything for you. If you want to get rid of the copy buttons use add this section too:

"pluginsConfig": {
  "code": {
    "copyButtons": false
  }
}