Acquirement Method: Obtaining the Thief Manual involves a chain of events. Immediately upon entering Stage 5, check out the sign. Next, go to the next room and examine the well. Then, proceed to room three and inspect the dead tree. Afterward, return to the very first room and inspect the abandoned building. Defeat Randy the Thief and choose to let him live; in return, he'll gift you with the Thief Manual.
Function: Needed for the hidden chest in Mireno's Underground Lake, which contains a Tarot x5. Refer to this guide for details on how to reach it.