Short answers, pulled from the story.
P.S. I Love You was composed principally by Paul McCartney and credited to the Lennon-McCartney partnership. John Lennon acknowledged it was mainly McCartney's song, saying McCartney was trying to write something like "Soldier Boy" by the Shirelles.
The version on the single and debut album was recorded on the 11th of September 1962 at EMI's Abbey Road Studios in London, completed in ten takes. The Beatles also recorded the song for BBC radio on the 25th of October 1962, the 27th of November 1962, and the 17th of June 1963.
Producer George Martin had booked session drummer Andy White as a replacement for Pete Best, not knowing the Beatles had already replaced Best with Ringo Starr. With White on drums, Starr attended the session and played maracas instead.
Producer Ron Richards told Paul McCartney the song could not be the A-side because an earlier record already existed with the same title. Richards, who came from a music publishing background, knew of that prior release and assigned the song to the B-side.
P.S. I Love You first appeared in the United States on Introducing... The Beatles in 1964. It was later included on the 1965 reissue The Early Beatles and the 1977 compilation Love Songs.
Music critic Ian MacDonald identified a sudden shift to B-flat underneath the song's title phrase as a "dark sidestep." He also noted that the chord D-flat-7 appears incongruously between G and D during the opening chorus.