Living More With Less, by Doris Janzen Longacre
Rich Christians in an Age of Hunger, by Ronald J. Sider

Both of these are from a Mennonite Christian perspective and both are somewhat dated by now (they came out in the 1970s.) Living More With Less has a lot of practical suggestions while Rich Christians in an Age of Hunger presents a religious argument for living simply.

How to Survive Without a Salary, by Charles Long

Long covers practical items such as finding work, buying at auctions, budgeting, clearing off debt and more.