Answer by taking a quote from the following article:

Simon & Garfunkel were an American folk rock duo consisting of singer-songwriter Paul Simon and singer Art Garfunkel. They were one of the best-selling music groups of the 1960s and became counterculture icons of the decade's social revolution, alongside artists such as the Beatles, the Beach Boys, and Bob Dylan. Their biggest hits--including "The Sound of Silence" (1964), "Mrs. Robinson" (1968), "The Boxer" (1969), and "Bridge over Troubled Water" (1970)--reached number one on singles charts worldwide. Their often rocky relationship led to artistic disagreements, which resulted in their breakup in 1970.

The duo's fourth studio album, Bookends, was recorded in fits and starts over various periods from late 1966 to early 1968. The duo were signed under an older contract that specified the label pay for sessions, and Simon & Garfunkel took advantage of this indulgence, hiring viola and brass players, as well as percussionists. The record's brevity reflects its concise and perfectionist production. The team spent over 50 studio hours recording "Punky's Dilemma", for example, and re-recorded vocal parts, sometimes note by note, until they were satisfied. Garfunkel's songs and voice took a lead role on some of the songs, and the harmonies for which the duo was known gradually disappeared. For Simon, Bookends represented the end of the collaboration and became an early indicator of his intentions to go solo. Although the album had been planned long in advance, work did not begin in earnest until the late months of 1967.  Prior to release, the band helped put together and performed at the Monterey Pop Festival, which signaled the beginning of the Summer of Love on the West Coast. "Fakin' It" was issued as a single that summer and found only modest success on AM radio; the duo were much more focused on the rising FM format, which played album cuts and treated their music with respect. In January 1968, the duo appeared on a Kraft Music Hall special, Three for Tonight, performing ten songs largely culled from their third album. Bookends was released by Columbia Records in April 1968. In an historical context, this was just 24 hours before the assassination of civil rights movement activist Martin Luther King Jr., which spurred nationwide outrage and riots. The album debuted on the Billboard Top LPs in the issue dated April 27, 1968, climbing to number one and staying at that position for seven non-consecutive weeks; it remained on the chart as a whole for 66 weeks. Bookends received such heavy orders weeks in advance of its release that Columbia was able to apply for award certification before copies left the warehouse, a fact it touted in magazine ads. The record became the duo's best-selling album to date: it fed off the buzz created by the release of The Graduate soundtrack album ten weeks earlier, creating an initial combined sales figure of over five million units.  Davis had predicted this fact, and suggested raising the list price of Bookends by one dollar to $5.79, above the then standard retail price, to compensate for including a large poster included in vinyl copies. Simon instead scoffed and viewed it as charging a premium on "what was sure to be that year's best-selling Columbia album". According to biographer Marc Eliot, Davis was "offended by what he perceived as their lack of gratitude for what he believed was his role in turning them into superstars". Rather than implement Davis' price increase plan, Simon & Garfunkel signed a contract extension with Columbia that guaranteed them a higher royalty rate. The lead single "Mrs. Robinson" became, at the 1969 Grammy Awards the first rock and roll song to receive Record of the Year; it was also awarded Best Contemporary Pop Performance by a Duo or Group.

what did they do in the studio?
The duo's fourth studio album, Bookends, was recorded in fits and starts over various periods from late 1966 to early 1968. The duo were signed under an older contract