With Sebastian gone and the book now in your hand, the best option you have would be to talk to Professor Fig to find out more.
It seems that the book has more secrets than meets the eye and you will need to discuss it with Professor Fig.
During this quest, more is learned about the book but you and the professor will not be investigating any time soon since he has been called elsewhere.
How To Get Tomes and Tribulations?
Tomes and Tribulations become available just after you have completed, Secrets of the Restricted Section quest.
Tomes and Tribulations Quest Walkthrough
1) Return to Professor Fig’s Classroom
Beginning where you left off in the Library, you will now have the book that you were searching for and need to take it back to Professor Fig.
He will be waiting in his classroom and you will need to go there by walking or fast traveling to speak with him about what happened.
Once you are in Professor Fig’s classroom, there will be an area that you need to interact with to proceed with the quest.
2) Take the Book to Professor Fig
You will need to make your way to Professor Fig, who will be having a conversation with Professor Sharp, who shortly leaves when you arrive.
Once Professor Sharp leaves, walk up to Professor Fig and speak with him to begin a cutscene where the two of you discuss the book and what happened.
Professor Fig will go through the book during a cutscene and notice that some of the pages have gone missing, meaning someone goes to it first.
As the cutscene ends, you are first told to continue with your studies, which mainly include Herbology and Potions.
Tomes and Tribulations Rewards
Tomes and Tribulations quest is more of an interlude between your quests and does not provide you with any rewards after being completed.
Professor Fig appears to be quite busy this time but you were still able to show him the book that you had found.
It looks like the professor will not be around for quite a while and it is time to continue with your classes as he instructed.
While the book is now in hand, you will need to learn more to be more adept while Professor Fig is away.