You can buy Item Maps for each area which will reveal the location of Collectibles.
#1: Sato's Letter
#2 Hyogo's Letter
#3: Mori's Letter
#4: Ren's Letter
#5: Mogi's First Letter
6: Mogi's Second Letter
7: Mogi's Third Letter
#8: Mogi's Fourth Letter
#9: Tadao's First Letter
#10: Tadao's Second Letter
#11: Shinji's Letter
#12: Hayato's First Letter
#13: Hayato's Second Letter
#14: Hayato's Third Letter
#15: Hayato's Fourth Letter
#16: Masahiro's Letter
#17: Hurk's First Letter
#18: Hurk's Second Letter
#19: Hurk's Third Letter
#20: Hurk's Fourth Letter
It says it reveals a secret wats the secret?