There's lots of classics, although I don't know where to find a list. Keep in mind it has been a long time since I've read any of these.
Some classics are:
Power Through Prayer by E.M. Bounds
My Utmost For His Highest by Oswald Chambers (a daily devotional which teaches a lot of good stuff)
The Practice of the Presence of God by Brother Lawrence
May be classic or could be:
The Grace Awakening by Charles Swindoll (a must read for many Christians; there's also a good workbook for it)
Classic Christianity by Bob George
The Master Plan of Evangelism by Robert Coleman
Breaking the Prayer Barrier by Michael Baughen
How to Read the Bible for All its Worth by Fee & Stuart
Victory Over the Darkness by Neil T. Anderson
Probably more than one by C.S. Lewis (few of which I've read, but others could recommend specific ones)
I've only read 2 or 3, but probably anything by Larry Crabb (psychologist).