After celebrating its 50^th anniversary just a few months ago, Kepler’s Books suddenly closed its doors yesterday. What a shock, and what a shame — it was one of the most prominent independent bookstores in the U.S., akin to City Lights in San Francisco, Vroman’s in Pasadena, or Powell’s in Portland. Situated in Menlo Park near Stanford, Kepler’s had a long storied history. It was another victim of the economic downturn and the spread of chain and online bookstores.