Well now, one of my all-time favorite movies, The Ninth Gate (1999), and the short novel it is based on, The Club Dumas.
Anyway, according to this site, the transcript shows thusly:
YOU HAVE SOME VERY RARE EDITIONS HERE.
ARE YOU SURE YOU WANT TO SELL THEM ALL?
THEY'RE NO USE TO FATHER, NOT ANYMORE...
NOT SINCE HE'S BEEN THIS WAY.
HIS LIBRARY WAS HIS WHOLE WORLD.
NOW IT'S JUST A PAINFUL MEMORY.
WELL, AT A ROUGH PRELIMINARY ESTIMATE...
YOU HAVE A COLLECTION HERE... WORTH AROUND $600,000.
YES, OR THEREABOUTS.
I'VE PICKED OUT ONE OR TWO VOLUMES THAT MERIT SPECIAL ATTENTION.
THIS "PERSILES", FOR EXAMPLE, IS IMPORTANT. I'D HANG ON TO THAT.
IT WILL NEVER DEPRECIATE. IT'S A GOOD INVESTMENT.
So, from there I ended up in the Wikipedia entry for The Club Dumas, which is a 1993 novel by Arturo Pérez-Reverte. It is also the novel that Roman Polanski used to base the screenplay for this movie on.
Down the entry there is a section called Literary References, Real Books, Other works mentioned, which includes:
Here is an English translation of this book. The book itself, was completed three days before de Cervantes died.
As it turns out, Miguel de Cervantes also wrote Don Quixote, which is the four book set Dean Corso actually buys from Telfer's son and daughter-in-law. So it would make sense that the Los trabajos de Persiles y Sigismunda would be in the same personal library of the ailing (deceased, in the novel) Andrew Telfer.
My only issue here, is that none of the scripts I can find online include that line. So, either Polanski added ad-hoc, Depp adlibbed it, or it's in the final script, which I can't locate.
But, I think that is what is being referred to here.