<?php
  require_once 'Google/Client.php';
  require_once 'Google/Service/Books.php';
  $client = new Google_Client();
  $client->setApplicationName("Client_Library_Examples");
  $client->setDeveloperKey("YOUR_APP_KEY");
  $service = new Google_Service_Books($client);
  $optParams = array('filter' => 'free-ebooks');
  $results = $service->volumes->listVolumes('Henry David Thoreau', $optParams);

  foreach ($results as $item) {
    echo $item['volumeInfo']['title'], "<br /> \n";
  }