The librarian has a list of books you're not allowed to take out. You request one of those books (book X), but it takes a while for search to run to see whether you're allowed to or not. While you're waiting, you say "actually, I'm not really interested in taking out book X, but if the content of that book is 'a', I'd like to take out book Y. If the content of that book is 'b', I'd like to take out book Y+1, and so on".

The librarian is still waiting for the search to complete to see if you can take out book X, but doesn't have anything better to do, so looks inside it, sees that the letter is 'b', and goes and gets book Y+1 so she can hand it over to you.

Now, the original check to see if you can take the first book out completes, and the librarian says "I'm sorry, I can't let you have book X, and I can't give you the book I fetched that you are allowed to take out, otherwise you'd know the content of the forbidden book."

Now, you request book 'Y', which you are allowed. The librarian goes away for a few minutes, and returns with book 'Y', and hands it over to you. You request book 'Y+1', and she hands it over immediately. You request book 'Y+2', and she goes away for a few minutes again, and hands it over.

You now know that Y+1 was (probably) the book she fetched when you made the forbidden request, and therefore that the letter inside the forbidden book was 'b'.