"Little by Little" is a song of mourning. The title, repeated throughout the song, refers to Plant's process of gradually coming to grips with a loss and finding himself able to move on. Little by little, he's healing from the trauma of a loss.
Because of Plant's status as legendary music figure and because "Little by Little" was the most successful single off his Shaken 'n' Stirred album, many fans and journalists have tried to pin down who or what Plant is mourning in the song. The most frequent suspects are drummer John Bonham (whose 1980 death effectively ended Led Zeppelin), Plant's son Karac (who died at just five years old in 1977), or Led Zeppelin itself. Another candidate could be Plant's wife Maureen, who he divorced just two years prior to releasing Shaken 'n' Stirred.
The lyrics, though, are quite vague and give no reason to assume they are about any specific person. One verse even suggests that the person being missed is Plant himself (as in some kind of spiritual or psychological self-identification):
Back at the mirror, your good friend
Talk to the mirror to play out your game
Slap in the middle, I stop then
Look at the winner and the price you pay
Plant wrote the song with Gerald "Jezz" Woodroffe, who played keyboards on all of Plant's first three solo albums. He also co-wrote many of the songs off those albums, either with Plant or with Plant and Robbie Blunt.
The song was featured in a Miami Vice episode titled "Junk Love." This was a significant event at the time, as Vice was the era's most influential television show and was known for its soundtracks. It used popular music rather than scored (and usually generic-sounding) music that was standard in '80s TV. Often, publications like TV Guide would preview and promote the show's song selection along with the episode itself.
"Little by Little" was a success on an album otherwise considered to be a failure by many music historians. Shaken 'n' Stirred peaked at #20 in America, but it didn't sell half as many copies as Plant's first two solo albums, Pictures at Eleven and The Principle of Moments. Even worse was the tour, which found Plant performing to venues that not only failed to sell out but were sometimes not even close to doing so.
Much of the album sounds chaotic, at times more like a strange sound experimentation than an album intended to sell. Many have tied all of this to Plant's life at the time, as he had recently left his wife of 15 years for her sister, who also happened to be the partner of the man who managed Plant's farm estate.
Author Dave Thompson argues against those common interpretations in Robert Plant: The Voice that Sailed the Zeppelin, pointing out that Plant's divorce and his new relationship weren't as crazy and tumultuous as they may seem to outsiders, and that Plant may have simply wanted to experiment with some wilder music.
Plant himself never characterized the events as chaotic at all. In fact, his wife Maureen soon moved back to the estate while Plant got a home nearby. They all remained on good terms.
Whatever the case, the album was not only a relative (to expectations) disaster in terms of quality and sales, it also destroyed the band that had backed Plant on his first three solo albums. According to Woodroffe's testimony in Robert Plant: A life, Robbie Blunt, the guitarist for that backing band and the cowriter of nearly every song from the first three albums, didn't want to be part of what Plant was doing anymore.
In the same book, Benji Le Fevre, who produced Shaken 'n' Stirred, supported Woodroffe's memory and also added a musing to support the notion that the album reflected Plant's inner state. "Everything was emotionally confused," Le Fevre says in the book. "It was going on at the same time that he was taking up with Shirley." Shirley was Shirley Wilson, sister of Maureen Wilson, Plant's ex-wife.
In addition to hitting #36 on the Hot 100, "Little by Little" held #1 on the Mainstream Rock chart for two weeks.