Information/Device For Gaining Superpowers In Gotham City Public Library

The Mars Conspiracy is a novel in the library that explains how to build a cybernetic assassin. While not a conventional power, it is something that could be used to gain power.

The Mars Conspiracy is a novel created by Simon Sherwood, published by Gotham City: Berryman Books, 2004.

The book contained revelations about a secret plan to design and build a cybernetic assassin known as Project Mars. The author, who helped design the machines artificial intelligence, tried to use the book to expose the shadowy figures behind the project however they sent one of the assassins to destroy the book during its lunch. The author, editors and printer were killed as well as the publisher's office being blown up and the distributor's warehouse burned to the ground before Batgirl and Oracle were able to find one of the book's last copies in the Gotham City Public Library and stop the cyborg. However the last copy was destroyed when the cyborg self destructed.

As Valorum alluded to in a comment, "Bats, Books, and Crazy Crooks", in Batman '66 #18 has Bookworm seeking out an occult book.

Entitled "Bats, Books and Crazy Crooks," this Jeff Parker-written, Richard Case-drawn comic is a lot of fun. Bookworm and his beatnik henchmen muscle their way in to the Gotham Public Library at closing time in an attempt to steal its rarest book, The Cleoveritas, "a repository of occult knowledge." His plan is to force Gordon to open the display case for him, but she runs off into the stacks, in order to transform into her crimefighting alter-ego.

And apparently, the book works.