Solution

1. Find the librarian.
2. Look at librarian, if she's carrying a riddle paper someone else has solved the quest too recently and you will need to wait an hour or so.
3. If the librarian doesn't have paper, start the quest with

   say give me the riddle

4. The librarian should respond:

   Librarian says: Actually I could use your help with this riddle...
   Librarian gives you her riddle.
   Librarian says: Search for the answer in the book of riddles, I can't find it ! Then come back and give me back the paper.

5. check your inventory, you should have “riddle paper”.
6. read the paper with “read paper”, it will have a riddle on it. Example

   At night they come without being fetched,
   And by day they are lost without being stolen,
   What are they?
   
   When you find out the answer, return this paper
   to the Librarian, and then tell her the answer.

7. Explore the library. The location of the book changes every hour or so. Even if you know the riddle answer you must find the book to solve the quest.
8. In each room try

   search book

9. Eventually you will find it:

   You search around and find a book !
   You take the book.

10. Check your inventory, you should have “A dusty book entitled 'The Book of Riddles'”.
11. Read the book with

    read book

12. It will have the riddle answer, e.g.

    You start reading the book attentively and suddenly notice
    you might have found the answer to your riddle :
    
    At night they come without being fetched,
    And by day they are lost without being stolen,
    What are they?
    
    Answer: Stars.

13. Go back to the librarian.
14. Give the librarian the paper with

    give paper to librarian

15. She should respond:

     You give your riddle to Librarian.
    Librarian asks: Hurray ! Now, what's the answer to this riddle ?

16. Now, say the answer to the riddle you found in the book, e.g.

    say stars

17. She should respond:

    Librarian exclaims: Thank you, <YourName> !!!
    She takes the paper and hides it under her aba.
    You feel more experienced !

18. That's it! Congratulations (cheater ).