Especially "free" books. I got my latest debit card reward in the form of an Amazon gift card and used it to buy three books:
The Upside-Down Kingdom
A Meal with Jesus: Discovering Grace, Community, and Mission around the Table (RE: Lit)
The Normal Christian Life
Free books are fun! Thanks to the lobbyists for Wal-Mart defeating the lobbyists for banks, the government is mandating lower interchange rates on card transactions and thus my debit card no longer will earn rewards. Bummer.