The Catechism of the Catholic Church (also available online here and here and here)
The Compendium of the Catechism of the Catholic Church.(also available online here and here)
The Catholic Catechism and The Faith: A Question-and-Answer Guide to the Catechism of the Catholic Church by Fr. John Hardon.
This is the Faith by Canon Ripley.
My Catholic Faith by Bishop Morrow
The Catechism of the Council of Trent (also available online here and here)