Several possibilities come to mind:
Dolores could have read Emily’s book while she strolled through the human library; it is not likely with our present knowledge that she would have bothered to do so on her own but those two might have a history: Emily was William’s daughter, and while she told us that she preferred the Raj-simulation, she likely accompanied her dad to Westworld as well, she might even have started there.
A meeting with Dolores in Westworld is not unlikely - and if it happened, it would have been significant, since young Emily would have likely already known her from the real world.
Is it possible that her reaction had an impact on Dolores, maybe even re-awoke her once again?
She was, after all, one of Ford’s primary test subjects to find out what triggers a self-conscious AI; he would not have missed the opportunity.
It is also possible that a back-up exists of all human data. What company would fail to secure its core data? [Yes, less likely given Delos’ interest in a copy of that data - and Ford’s interest in keeping it from them.]
Ford or Bernard might have also created back-ups of key-individuals and hit them elsewhere - Ford might still be elsewhere as well, maybe even with those back-ups.
Bernard could also have been the key to recreate Emily from memory - if he ever read her book (he did visit the library repeatedly).
Less likely but also not impossible is the option that the data was recovered by Delos; as we know, no digital data is deleted as long as it is not overwritten or the medium is destroyed.
Of course, the post-credit-scene could also reveal that William is trapped within a variant of Roko’s basilisk.