History: Early Years Of The Beatles

Musical style and evolution
In Icons of Rock: An Encyclopedia of the Legends Who Changed Music Forever, Scott Schinder and Andy Schwartz sum up The Beatles' musical evolution:
In their initial incarnation as cheerful, wisecracking moptops, the Fab Four revolutionized the sound, style, and attitude of popular music and opened rock and roll's doors to a tidal wave of British rock acts. Their initial impact would have been enough to establish the Beatles as one of their era's most influential cultural forces, but they didn't stop there. Although their initial style was a highly original, irresistibly catchy synthesis of early American rock and roll and R&B, the Beatles spent the rest of the 1960s expanding rock's stylistic frontiers, consistently staking out new musical territory on each release. The band's increasingly sophisticated experimentation encompassed a variety of genres, including folk-rock, country, psychedelia, and baroque pop, without sacrificing the effortless mass appeal of their early work.
In The Beatles as Musicians, Walter Everett points out Lennon and McCartney's contrasting motivations and approaches to composition: "McCartney may be said to have constantly developed—as a means to entertain—a focused musical talent with an ear for counterpoint and other aspects of craft in the demonstration of a universally agreed-upon common language that he did much to enrich. Conversely, Lennon's mature music is best appreciated as the daring product of a largely unconscious, searching but undisciplined artistic sensibility."