Sergey let you in to the library.
You have to pull five switches in the book shelf on the far right.
The hints are:
This is the low-spoiler guide for "Book switches" puzzle in Chinatown Detective Agency. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.