"The Mortification of Sin" by John Owen * Where my profile name comes from, this little Puritan gem is very practical in our common struggle against sin -- the World, the devil, and our own Flesh.
"The Christian Life" by Sinclair Ferguson * This little book is a great theology, and the beauty of it is that the author still realizes theology must be lived, not just theorized. Ferguson is one of very few contemporary authors who can balance deep theology with practical living.