Group and sort results by max and count in joined table using QueryBuilder

I'm making an app for selling books but I'm struggling with this part of it.

When a user wants to buy a book he can send a message to the book's seller. I have a table called conversations and another table called messages, which holds the messages for a conversation.

I have a page where a user can see a list of conversations he's involved. Conversations with the latest messages should be shown first. I solved this already but I also want to show the number of unread messages next to each conversation (messages have a "read" boolean field). I haven't been able to add the count as part of my original query; I know I could make a query for each item in the conversations collection but that would be inefficient as hell.

This is my query so far with the QueryBuilder:

$conversations = DB::table('conversations')
            ->select([
                'books.title as bookTitle', 'conversations.id',
                 DB::raw('max(messages.created_at) as lastMessage'),
                // DB::raw('(select count(id) from messages')
            ])
            ->where('conversations.from_user', $this->user->id)
            ->orWhere('conversations.to_user', $this->user->id)
            ->join('books', 'conversations.book_id', '=', 'books.id')
            ->join('messages', 'conversations.id', '=', 'messages.conversation_id')
            ->groupBy('messages.conversation_id')
            ->orderBy('lastMessage', 'DESC')
            ->get();

If i add the part commented out to attempt to get a field with the number of unread messages for that conversation I get a SQL syntax error. I also been thinking if maybe is not possible to add the count field since it might be exclusive to my original query: I sort the conversations taking into account all messages (read or unread) and pick the latest message for each conversation; the count field I want to add should only count unread messages.

Any ideas? Hope I explained myself.

Thanks.

via Chebli Mohamed