Pierian spring

(idiomatic, chiefly literary) The source of knowledge, inspiration, or learning.

Example: 1711, Alexander Pope, An Essay on Criticism:
  A little Learning is a dang'rous Thing;
  Drink deep, or taste not the Pierian Spring.
1817, Samuel Taylor Coleridge, Biographia Literaria, ch. 1:
  At school, (Christ's Hospital,) I enjoyed the inestimable advantage of a very sensible, though at the same time, a very severe master, the Reverend James Bowyer. . . . [H]e showed no mercy to phrase, metaphor, or image, unsupported by a sound sense, or where the same sense might have been conveyed with equal force and dignity in plainer words. . . . In fancy I can almost hear him now, exclaiming "Harp? Harp? Lyre? Pen and ink, boy, you mean! Muse, boy, Muse? Your nurse's daughter, you mean! Pierian spring? Oh aye! the cloister-pump, I suppose!"
1892, Ambrose Bierce, "A Poet's Father" in Black Beetles in Amber:
  . . . a studious land
  Where humming youth, intent upon the page,
  Thirsting for knowledge with a noble rage,
  Drink dry the whole Pierian spring
2009 Jan. 2, Timothy W. Ryback, "First Chapter: Hitler’s Private Library," New York Times (retrieved 9 Aug 2015):
  For him the library represented a Pierian spring. . . . He drew deeply there, quelling his intellectual insecurities and nourishing his fanatic ambitions.