MongoDB Convert aggregate query in node.js to LINQ

I have an aggregate query in node.js that I'm struggling to convert to LINQ. Here is the original

const books = await collection.aggregate()
    .group({
        _id: "$author",
        count: { $sum: 1 },
        return_date: { $last: "$return_date" },
        title: { $last: "$title" },
    })
    .sort({ count: -1, _id: 1 })
    .limit(20).toArray();

The best I've been able to do so far is

IMongoQueryable<Popular> query = (from book in booksCollection.AsQueryable()
                                  group book by book.author into g
                                  select new Popular
                                  {
                                      Author = g.Key,
                                      Count = g.Count()
                                  }).OrderByDescending(g => g.Count).ThenBy(g => g.Author).Take(20);

Where Popular is

 public class Popular
 {
     public string? Title { get; set; } = null;
     public string? Author { get; set; } = null;
     public string? ReturnDate { get; set; } = null;
     public int? Count { get; set; }
 }

I can't find a way of including ReturnDate and Title, so I'm having to make an extra query for every result to add these fields. Is there a more elegant way of converting this query?

You can use the GroupBy and projection feature of LINQ to get ReturnDate and Title within one single LINQ query without firing an extra query

IMongoQueryable<Popular> query = (from book in booksCollection.AsQueryable()
                                  group book by book.Author into g
                                  let lastBook = g.OrderByDescending(b => b.ReturnDate).FirstOrDefault()
                                  select new Popular
                                  {
                                      Author = g.Key,
                                      Count = g.Count(),
                                      ReturnDate = lastBook.ReturnDate,
                                      Title = lastBook.Title
                                  })
                                  .OrderByDescending(g => g.Count)
                                  .ThenBy(g => g.Author)
                                  .Take(20);