Eddie Janssens · CC BY 3.0

Mark Lewisohn built his career from a teenage fan letter — after winning a quiz at London's first Beatles convention, he was hired to answer the band's fan mail, and turned that access into decades of primary research now considered the definitive record of the group's history. His Complete Beatles Recording Sessions reconstructed every day the band spent at Abbey Road from the studio's own logs, and since 2013 he has been publishing All These Years, a three-volume biography aiming to document the Beatles more thoroughly than any account before it.