*POSSIBLE SPOILER*
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After you get the book, visit Manami's shop and at some point she will call you and tell you what to do.
This gives you access to the puzzle that unlocks the secret ending, although there is no credits screen that says "Secret Ending", but it does unlock pictures. (And once you finish the game with the secret done, and start a new game, you can repeat it up to 2 more times).