It's rather quintessential, but The Ladder of Divine Ascent by St. John Klimakos.
I've been attending the Orthodox Church for nearly four years, and have been baptized for two, and I've never read it.
I don't know if I'll ever bring myself to read it. There's something off-putting about reading this sort of monastic literature.
Haha. I was blessed to read The Arena
by St. Ignaty Branchininov some time ago and thoroughly enjoyed it. I'm told the work is a summation of St. John's Ladder
I think we have to understand such works for what they are, literature written for a specific audience at a specific time, which means we aren't that audience anymore. There are great things about living spiritually in them, but we have to realize we aren't monastics, not even novices, and much less aged hermits! Not everything is going to apply to us in some direct, one-to-one style extrapolation.