What if an object's fields is an object...is there a way to get the latter object's fields?

I'm writing a program wherein I need to check an object's field values. For example, I have a class called Book which has String bookName, String publisher, and Author author. In Author, it has fields such as String firstName, String lastName, String dateOfBirth. I want to get the author's last name and change it to a new last name by looking through the fields of the Book. Also, I have to implement my method in a way wherein any kind of object can be used so method can also be used when changing first name of just the author.

I have used getDeclaredFields() to loop through the fields of my Book object, and am trying to find if one of the fields is its own object.

I have used "instanceof" Author but I need to find a way that doesn't need my hardcoding of the class name "Author"

public ArrayList<String> makeChanges(Object book){
ArrayList<String> changeList= new ArrayList<String>;
try{
Field[] bookFields = book.getClass().getDeclaredFields();

String changes = "";

for (Field field: bookFields){
    field.setAccessible(true);
    if (field.getName().equals("firstName")){
        if (!(field.get(book).equals("Twilight"))){
            changes = field.getName() + changed to "Twilight";
            changeList.add(changes);
        }
    if (field.get(book) instanceof Author){
        List<String> authorChanges = makeChanges(field.get(book));
        changes = Arrays.toString(authorChanges.toArray()).replace("[","").replace("]","");
        changeList.add(changes);
    }
}

catch(Exception e){
System.out.println(e.toString());
}

return changeList;
}

Actual result: [bookName changed to "Twilight", author changed to "demo.Address@16f65612"]

Expected result should be: [bookName changed to "Twilight", firstName changed to "Eli"]