You should be able to talk to the librarian to get back into the library and read the summoning book, though you do not need to.
The book the instructor is after is the Strategies of Summoning. Talk to the librarian about it, then follow the leads (or just talk to everyone until you get a clue where it is).
In detail, <span class='standouttext'>Spoiler : </span><span class='spoiler'>After visiting the library and talking to Antopar, you will find the book was lent to Belehram who gave it to her son Dolehar the drug addict. You should only need to talk to Dotharhar, though, who is in or near the component clerk's room. He tells you an imp may have the book. After that Tax should give you the key to the room his uncle Nixfix locked so you can enter that part of the maintenance tunnel to get the book.</span> (highlight the text to read)
