CliFx/CliFx.Demo/Domain/Library.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace CliFx.Demo.Domain;

public partial class Library
{
    public IReadOnlyList<Book> Books { get; }

    public Library(IReadOnlyList<Book> books)
    {
        Books = books;
    }

    public Library WithBook(Book book)
    {
        var books = Books.ToList();
        books.Add(book);

        return new Library(books);
    }

    public Library WithoutBook(Book book)
    {
        var books = Books.Where(b => b != book).ToArray();

        return new Library(books);
    }
}

public partial class Library
{
    public static Library Empty { get; } = new(Array.Empty<Book>());
}