Objects are not valid as a React child (found object)

// model =====================================
const mongoose = require('mongoose');

const Schema = mongoose.Schema;
const BookSchema = new Schema({
    title: { type: String },
    category: { type: mongoose.Schema.ObjectId, ref: 'Category' },
    pages: { type: Number },
    author: { type: mongoose.Schema.ObjectId, ref: 'Author' },
    published: { type: Date }
}, { timestamps: true });

module.exports = mongoose.model('Book', BookSchema);

// mongo query  ===============================
Book
 .find({}).sort({ title: 1 })
 .populate('author')
 .populate('category')
 .exec((err, results) => {
    res.json({ results: results });
 });

// reactjs ===================================
contructor() {
 super();

 this.state = { dataTable: {}, authorization: 'token' };
}

loadDataFromServer() {
  let dataTable = {};
  const Authorization = this.state.authorization;

  axios
    .get(`${ROOT_URL}/books`, { headers: { Authorization } })
    .then((res) => {
        // initialize dataTable value & change state
        dataTable = res.data.results;
        this.setState({ dataTable });
    });
}


// response example  ===================================

**i want to get author id & fullname from react
{
 "results": [
        {
            "_id": "5a6ea21602415a12bfd5f7af",
            "updatedAt": "2018-01-29T04:24:54.317Z",
            "createdAt": "2018-01-29T04:24:54.317Z",
            "published": "2010-12-11T17:00:00.000Z",
            "author": {
                "_id": "5a570490f086910c55bbbba9",
                "email": "marius@mariusgabriel.com",
                "fullname": "Marius Gabriel",
                "updatedAt": "2018-01-11T06:24:05.934Z",
                "createdAt": "2018-01-11T06:24:05.934Z",
                "__v": 0
            },
            "pages": 123,
            "category": {
                "_id": "5a56e8bfd80ed230a66ba010",
                "category_name": "Literature",
                "updatedAt": "2018-01-11T04:28:03.458Z",
                "createdAt": "2018-01-11T04:28:03.458Z",
                "__v": 0
            },
            "title": "Book IV",
            "__v": 0
        },
        {
            "_id": "5a6ea23b02415a12bfd5f7b0",
            "updatedAt": "2018-01-29T04:25:31.113Z",
            "createdAt": "2018-01-29T04:25:31.113Z",
            "published": "2002-01-02T17:00:00.000Z",
            "author": {
                "_id": "5a572bb0fea5b82f322d36c2",
                "email": "robi@robi.com",
                "fullname": "Robi",
                "updatedAt": "2018-01-18T10:53:51.828Z",
                "createdAt": "2018-01-11T08:27:55.183Z",
                "__v": 0
            },
            "pages": 222,
            "category": {
                "_id": "5a56e8d4d80ed230a66ba012",
                "category_name": "Fantasy",
                "updatedAt": "2018-01-11T04:28:03.458Z",
                "createdAt": "2018-01-11T04:28:03.458Z",
                "__v": 0
            },
            "title": "Book II",
            "__v": 0
        }
    ]
}