Go ahead and make a custom ModelAdmin for Author and Topic also:
class AuthorAdmin(admin.ModelAdmin):
search_fields = ['firstname', 'lastname']
ordering = ['lastname', 'firstname']
admin.site.register(Author, AuthorAdmin)
class TopicAdmin(admin.ModelAdmin):
list_display = ['title', 'slug']
search_fields = ['title', 'slug']
ordering = ['-changed']
prepopulated_fields = {'slug': ('title',)}
admin.site.register(Topic, TopicAdmin)