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_name"": ""The Dragon"",

              ""book_type"": ""stories""

            },

            {

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

              ""book_name"": ""Mirage"",

              ""book_type"": ""stories""

            },

            {

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

              ""book_name"": ""The burning forest"",

              ""book_type"": ""stories""

            },

            {

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

              ""book_name"": ""Traditions of Latin America"",

              ""book_type"": ""culture""

            },

            {

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

              ""book_name"": ""Addis Ababa"",

              ""book_type"": ""culture""

            }

          ]";

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

        var groupedByUserName = (from DataRow dr in dts.Rows

            select new

            {

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

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

                book_type = dr["book_type"].ToString()

            })

            .GroupBy(x => new { x.user_name, x.book_type })

            .Select(x => new 

                    {

                        user_name = x.Key.user_name,

                        book_type = x.Key.book_type,

                        count = x.Count()

                    })

            .GroupBy(x => x.user_name)

            .Select(x =>

                    {

                        JObject jObj = new JObject();

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

                        jObj.Merge(JObject.FromObject(x.ToDictionary(y => y.book_type, y => y.count)));

                        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; }

}