<?php

$s = <<<JSON
{
    "books":[
        {
        "email":"test@test.te",
        "author":"John Name",
        "created":"2017-03-24 10:41:21"
        },
        {
        "email":"test@test.te",
        "author":"John Name",
        "created":"2017-03-24 10:41:21"
        },
        {
        "email":"test@test.te",
        "author":"John Name",
        "created":"2017-03-24 10:41:21"
        }
    ]
}
JSON;

$json = json_decode($s, true);

$json['books'][] = [
  'email' => 'foo',
  'author' => 'bar',
  'created' => '42',
];

var_dump(json_encode($json, JSON_PRETTY_PRINT|JSON_UNESCAPED_UNICODE));