A Catechism is defined as "a summary or exposition of doctrine and serves as a learning introduction to the Sacraments traditionally used in catechesis, or Christian religious teaching of children and adult converts." Crossway and The Gospel Coalition have put together a brand new catechism called The New City Catechism.
Tim Keller explains what this catechism is about:
For one week, Westminster Bookstore has a single copy of the catechism for 25% off. If you wish to purchase more, you get 5 copies for 50% each. If you are a church leader, and want to purchase this book for your congregation, you may purchase a case of 50 books from 62% off each copy.
If you wish to do a little more in depth reading of the catechism, there is The New City Catechism Devotional which is only $11.00 for one week as well.
The Gospel Coalition also has an album based on the catechism which you can download for free: