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"Direct observation of the object which is giving off the magical emanations would dramatically increase the understanding of the emanations themselves. In other words, open it up so we can take a look at it!"
If you'll remember, it's not a secret door, but an area of false books in the bookcase where Capizzio is searching. Cappy grasps the false books and pulls them out from the bookcase like a drawer. They are located on the bottom shelf, so when they are pulled out, it's just like opening a drawer under a bed, or something. Inside the drawer are several thick books.
Kerlan immediately notices that the script of the book is unusual in that it seems to be written in more than just one language. It is mostly in Draconic, but there are portions in Undercommon and Drow as well.