To find Europe’s best bike routes, a cycling writer asked the crowd

When it came time to put together "Cycling Atlas Europe," a new guidebook detailing 350 single-day rides across the continent, sports writer and cycling expert Claude Droussent, 65, knew where to turn: Strava, a fitness app that lets cyclists (and runners and hikers) record their routes with GPS and then share them with one another, and that calls itself "the largest sports community in the world."

Having Strava users validate the routes and vouch for their safety and appeal was key to the book, whose subtitle is "The 350 Most Beautiful Cycling Trips in Europe," Droussent said: "The Strava community was important because I didn't know all 350 routes by myself."

The ambitious aim of the "Atlas," published by Rizzoli this spring: to sate both serious cyclists searching for epic conquests and the multitudes looking for saddle-top journeys of discovery in this soaring...