Seen Reading is a simple, elegant idea that I wish I had thought of, except that I have no idea where the author finds the time. She spots someone reading on the subway or bus, or in the park, or in a restaurant, or whereever, notes the author, title and approximate page of the book, goes to the nearest bookstore and finds the passage, copies it, then writes a short paragraph in which she imagines the reader's own life and thoughts. It's fascinating, and somehow magical.
Her Harry Potter post captures the magic of THAT phenomenon very well. Don't worry - no spoilers.