Pass comparator into map with collectors

Question

I want to collect an enum into a treemap and I want to have case-insensitive map. From here I see https://www.baeldung.com/java-collectors-tomap how to return a treemap

public TreeMap<String, Book> listToSortedMap(List<Book> books) {
    return books.stream() 
      .collect(
        Collectors.toMap(Book::getName, Function.identity(), (o1, o2) -> o1, TreeMap::new));
}

but the natural ordering will be case-sensitive. How can I pass a custom comparator in the above return statement? Something that gives me as below

Map<String, Integer> treeMap = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);