I can add very little to the discussion about the lyrics of "Norwegian Wood" . They read more about a "one night stand" than an "affair" to me but who am I to say?
But Ian MacDonald has quite a lot to say and I would encourage you to read his book, "Revolution In The Head". Lennon started the song in St Moritz in January 1965 and McCartney made a contribution, as said in an earlier post, but MacDonald suggests the latter's contribution was significant ("close to a fifty-fifty collaboration").
MacDonald thinks that the music may have been influenced by a meeting that Lennon and Harrison had with two of The Byrds in August 1965, at which they discussed the sitar and ragas, whilst on a trip. Harrison's sitar is double-tracked. The song was not recorded until October 1965 and MacDonald thinks that the recording was varispeeded for the RUBBER SOUL release.
By way of background, MacDonald says that Lennon and McCartney "were aware that Bob Dylan, with his tumultuously original singles Subterranean Homesick Blues and Like A Rolling Stone, had rolled back the horizons of the pop lyric in a way they must acknowledge and somehow outdo". MacDonald says that Lennon told him that the other Beatles joshed him about copying Dylan. MacDonald goes on to describe "Norwegian Wood" as "the first Beatle song in which the lyric is more important than the music". He also says that "Lennon was uneasy about trespassing on Dylan's territory" and that, when Dylan parodied "Norwegian Wood" (with "4th Time Around") on his BLONDE ON BLONDE album, Lennon later admitte to being "paranoid" about it.
MacDonald says that, "Aside from a reference to the Sixties fashion for Scandinavian pine interiors, Lennon admitted to having no idea why NORWEGIAN WOOD was so-called". The sub-title of the song is "This Bird Had Flown" and this was apparently its early title.
