class Book
{
public string Name { get; set; }
public string Author { get; set; }
}
class User
{
public string Name { get; set; }
public int Age { get; set; }
public Book[] Books;
}
var us = Users.SelectMany(u => u.Books)
.Where(b => b.Author == "Достоевский")
.Distinct()
.Select(b => b.Name);