I loved it! It was an adventure minus the crazy government, vampires, zombies, whatsoever. :)
Here are the things I love about the book:
1. It has books- And bookstore, of course! Who doesn't love a book about books and bookstore?
2. Puzzles, codes- It got me curious with Manutius codex vitae. (I will not spoil anyone, so I'll stop here.):)
3. Characters!- I love Clay, Neel, Mr. Penumbra (of course), Daleck, and Mat! I like Kat's enthusiasm about her work but I disagree with some of her "principles." :)
4. The lesson of the story- which can be found in the Epilogue... Haha!
5. Programming- even though I did not understand them, it was interesting. Haha!
If you haven't read Mr. Penumbra's 24-Hour Bookstore, ugh guys, then you should pick it up soon. :)