First, ask at the information desk of the library. The general staff will collect books and other items left by students.

If there is no surveillance video to investigate the library, check your location and time, and you will know who took your things.

Then you can call the police according to the video or ask the school security office (if you are a student).

If you call the police, the police will also investigate the video. This is direct evidence and the best clue. If there is no video, I am afraid the difficulty will increase. Good luck!