List<SampleRecord> records = Lists.newArrayList();
Map<String,Set<String>> map = Maps.newHashMap();
.............................
records.add(new SampleRecord("G1","F1"));
records.add(new SampleRecord("G2","F2"));
records.add(new SampleRecord("G3","F3"));
records.add(new SampleRecord("G4","F4"));
records.add(new SampleRecord("G4","F5"));
records.add(new SampleRecord("G5","F6"));
records.add(new SampleRecord("G6","F7"));
records.add(new SampleRecord("G6","G4"));
records.add(new SampleRecord("G7","G3"));
records.add(new SampleRecord("G8","G7"));
..........................
//рекурсивная функция
public Set<String> findBook(String group){
        Set<String> books = Sets.newHashSet();
        records.stream()
                .filter(r -> group.equals(r.group))
                .map(SampleRecord::getFolder)
                .forEach(f -> {
                    if (isLast(f)){
                        books.add(f);
                    } else {
                        books.addAll(findBook(f));
                    }
                });
        return books;
}
//основной вызов
records.stream()
                .map(SampleRecord::getGroup)
                .distinct()
                .forEach(g -> map.put(g,findBook(g)));