ok la teoria, ma a livello pratico mi manca qualcosa:
codice:
    public Map<String, Long> countByAuthor(List<Book> booksList) { 
        Map<String, Long> map = booksList.stream().collect(Collectors.groupingBy(Book::getAuthor, Collectors.counting())); 
        Map<String, Long> sorted = sortByValues(map); 
        return sorted; 
    } 
 
    public <K, V extends Comparable<V>> Map<K, V> sortByValues(Map<K, V> map) { 
        Comparator<K> valueComparator = (k1, k2) -> { 
            int compare = map.get(k2).compareTo(map.get(k1)); 
            return (compare == 0) ? 1 : compare; 
        }; 
        Map<K, V> sortedByValues = new TreeMap<>(valueComparator); 
        sortedByValues.putAll(map); 
        System.out.println(map); 
        System.out.println(sortedByValues); 
        return sortedByValues; 
    }
stampando le due Map, vedo che me le ordina correttamente.
ora però dovrei usare limit(10) sulla Map sorted, giusto?