Integrating Mailjet API v3 wrapper as Codeigniter library

Yes, you are on right track. But you don’t need to create CI library. Use Mailjet repository library in controller as well. Just use composer as stated in CI docs.

If you want CodeIgniter to use a Composer auto-loader, just set $config[‘composer_autoload’] to TRUE or a custom path in application/config/config.php.

Step by step instruction for using github repository in CodeIgniter

1. Set $config['composer_autoload'] = TRUE; in APPPATH.'config/config.php' file
2. Put composer.json file with wanted repositories/projects in APPPATH location
3. Do the job with composer install command through console which will make vendor and other related files and folders inside
4. Use it when needed in controller or in other code as shown in example bellow

example controller Mailman.php

<?php defined('BASEPATH') OR exit('No direct script access allowed');

use \Mailjet\Resources;

class Mailman extends CI_Controller
{
    private $apikey = 'apy__key__here';
    private $secretkey = 'apy__secret__here';

    protected $mj = NULL;

    public function __construct()
    {
        // $this->mj variable is becoming available to controller's methods
        $this->mj = new \Mailjet\Client($this->apikey, $this->apisecret);
    }

    public function index()
    {
        $response = $this->mj->get(Resources::$Contact);

        /*
         * Read the response
         */
        if ($response->success())
            var_dump($response->getData());
        else
            var_dump($response->getStatus());
    }
}

If you explicitly want to use Mailjet (or any other) repository through CI library, check in docs how to create custom library and merge this code above with it. Personaly I use repositories this way to avoid unnecessarily loading and parsing sufficient libraries.