A letter of recommendation is required to enter the Hidden Library. Go to Brigade General Suthran in Altgard to get your letter.
Step 1. Go to SuthranThe Brigade General of Altgard Fortress. He’s quite brusque when he thinks his time is being wasted. to get a letter of recommendation.
Step 2. Talk with OublietteA gatekeeper guarding the door of the Hidden Library in the Temple of Knowledge in Pandaemonium..