Reformation Heritage Books is offering Daniel Hyde's Welcome to a Reformed Church: A Guide for Pilgrims for just $7.50 for this week only. I haven't read this one but it looks to be a good book that covers both what Reformed churches believe but also why they have the practices that they do. Sean Michael Lucas' On Being Presbyterian: Our Beliefs, Practices, and Stories is my favorite book in this category but Pastor Hyde has written some other excellent things and I'm sure this will be worthwhile.
Also, the Meet the Puritans blog has made a study guide available for John Owen's Communion with God. This would be a great book to work through for anyone who finished Calvin's Institutes of the Christian Religion last year. The guide is free as a .pdf here.