Mr. Penumbra’s 24-hour Bookstore by Robin Sloan Farrar, Straus and Giroux, $25.00, 288 pages, ISBN 978-0-374-21491-3 Do you ever wish you were a kid again?  To go back in time when learning was easy and quick for you?  When your biggest worry was coming up with your share of the rent?  Do you like quests; the hunt for the priceless artifacts that belong to all of humanity, but somehow