using System;

using System.Collections.Generic;

using System.Data;

using System.Linq;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"[

            {

              ""user_name"": ""Alex"",

            ""book_types"": [{""culture"":2},{""stories"":3}]

            }

          ]";

        DataTable dts = JsonConvert.DeserializeObject<DataTable>(json); 

        var groupedByUserName = (from DataRow dr in dts.Rows

            select new

            {

                user_name = dr["user_name"].ToString(),

                book_types = JArray.FromObject(dr["book_types"])

            })

            .Select(x =>

                    {

                        JObject jObj = new JObject();

                        jObj.Add("user_name", x.user_name);

                        foreach (JObject book_type in x.book_types)

                        {

                            jObj.Merge(JObject.FromObject(book_type));

                        }   

                        return jObj.ToObject<object>();

                    })

            .ToList();

        var apiResult = new ApiResult

        {

            Data = groupedByUserName,

            Count = groupedByUserName.Count,

            Success = true,

        };

        Console.WriteLine(JsonConvert.SerializeObject(apiResult, Formatting.Indented));

    }

}

​

public class ApiResult

{

    public List<dynamic> Data { get; set; }

    public int Count { get; set; }

    public bool Success { get; set; }

}