using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var ivan = new User() { Id = 1, Name = "Ivan" };

        var vasya = new User() { Id = 2, Name = "Vasya" };

        var book1 = new Book() { Id = 1, Name = "1" };

        var book2 = new Book() { Id = 2, Name = "2" };

        var book3 = new Book() { Id = 3, Name = "3" };

        var book4 = new Book() { Id = 4, Name = "4" };

        book1.Users.Add(ivan);

        book1.Users.Add(ivan);      

        book1.Users.Add(vasya);

        book1.Users.Add(vasya);

        book2.Users.Add(ivan);

        book2.Users.Add(ivan);      

        book2.Users.Add(vasya);

        book2.Users.Add(vasya);

        book4.Users.Add(ivan);

        var booksRepository = new List<Book>();

        booksRepository.Add(book1);

        booksRepository.Add(book2);

        booksRepository.Add(book3);

        booksRepository.Add(book4);

        var result = booksRepository.Where(b => b.Users.Any(u => u.Name == "Ivan")).ToList();

        foreach (Book book in result)

        {

            Console.WriteLine(book.Name);

        }

    }

    public class User

    {

        public long Id {get;set;}

        public string Name {get;set;}

        // List<Book> ...

    }

    public class Book

    {

        public long Id {get;set;}

        public string Name {get;set;}

        public List<User> Users {get;set;}

        public Book(){

            Users = new List<User>();

        }

    }

}