this is taken from E.A.Poe's The Purloined Letter
“Than whom,” said Dupin, amid a perfect whirlwind of smoke, “no more sagacious agent could, I suppose, be desired, or ever imagined.”within the context of this short story the sentence amounts to
" A task (,namely that of getting back the letter) for which you couldn't find a more capable person"
of course, Poe's English is both a bit dated and probably tailored to fit Mr.Dupin,a Frenchman , as I seem to recall.
Student or Learner