Google Books API for PHP >= 5.3.0. It currently only supports searching a specific book by ISBN.
$key = 'YOUR_API_KEY';
$query = '084930315X'; // 10 or 13 digits ISBN
$book = new GoogleBooks\GoogleBooks($key);
if ($book->searchByISBN($query))
{
// success
$title = $book->getTitle();
}
else
{
// error
}