Create an index with a sort condition

Hello,

I use PostgreSQL 8.1 and I would like create and index on a table's
field with a sort condition on this field.
For example, I have a table named books and I want create an index on
the fields title and id_book with an order by id_book descendant.

I know how to create the index without this condition :

create index book_index on books(id_book, title);

But, I don't know to create this index with the sort condition.
Someone has an idea ?

Thanks to help me.

Sylvain.