After an incredible celestial event, Twilight realizes there may be somepony who needs help. The search for this pony evolves into a much greater journey of forgotten metropolises, a dangerous unbalance of magic, and, most importantly, memory.
The direct sequel to A Rose is a Rose (though you need not have read that).
(ARIAR EQD link: http://www.equestriadaily.com/2011/09/story-rose-is-rose.html )