I've maintained that John Calvin's Institutes of the Christian Religion are an invaluable book that even lay people in the church should study and read. A new edition has recently been published that includes the full text in a single hardcover volume. It is available at Amazon for a $16.47 and at Reformation Heritage books for just $15. It does use the Henry Beveridge translation, which is a little older for modern readers and probably not quite as readable as the Ford Lewis Battles translation. On the other hand, it's also available for about $35 less.