Not actually a book, but a short story - but Goodreads has it listed all on its own...
This brief piece functions as a creation myth of the mysterious library featured in 'The Shadow of the Wind' and sequels.
It's a lovely, atmospheric piece, but not a major work. It's being pushed as a 'teaser' to the book series, but I'm not so sure that it works well as one, because it is really quite different from the novels on several levels.