score:1

var temp = ctx.books
  // probably some where clause
  .select(x => new
      {
        bookid = x.bookid,
          bookname = x.title,
          authors = x.authorbooks.select(a => a.author)
          })
  .toarray();

var books = temp.select(x => new { 
    x.bookid, 
    x.bookname, 
    authors = x.authors.aggregate(new stringbuilder(), (sb, r) => sb.append(r.firstname + " " + r.lastname +"& "), sb => sb.tostring()) 
});

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