Wouldn't it be nice to buy a Beach Boys song?

Sheet music, photos, and other lost treasures of the Californian group, go up for sale in London next month

The Beach Boys' songs of youth and romance won a worldwide audience and fame to rival the Beatles. Now an archive of material, spanning the first two decades of their career and revealing how some of the songs were created, is to be auctioned in London next month.

Discovered in a storage facility in Florida, the material is thought to be one of the biggest collections to be put up for auction and represents thousands of pages of manuscripts, covering more than 150 songs. It consists of recording contracts, sheet music, photographs and handwritten lyrics and letters.

Such is the vast size of the archive – revealed here for the first time – that a reserve of $10m (£6.6m) has been set. It will be one of the largest amounts paid for a single collection of rock'n'roll memorabilia if the asking price is achieved.

The group, who formed in 1961, had instant success with songs such as "Fun, Fun, Fun" and "Surfin' USA". By the mid 1960s, it was evident that leader Brian Wilson was an incredibly gifted songwriter, and their 1966 album Pet Sounds is often cited as one of the greatest albums ever recorded. On hearing the song "God Only Knows" for the first time, Paul McCartney called it "the greatest song ever written".

But by the late 1960s, Wilson's mental health deteriorated considerably and he became a recluse for much of the next decade, with his musical output proving patchy.

Wilson became addicted to cocaine and overeating, his weight ballooning during the late 1960s and his paranoia about producing material that would compete with the Beatles increasing.

The collection was discovered during a blind auction at a storage facility in Florida in 2000 and later sold to a private company as part of a larger sale after several years of prolonged legal disputes.

Alan Boyd, Beach Boys archivist and expert, said: "This historic collection, containing many of the Beach Boys' own publishing documents along with assorted handwritten musical pieces, vintage legal papers, and various promotional and personal items from their early years, presents a priceless look into the inner workings of this legendary group.

"Historical artefacts like Brian Wilson and Mike Love's signatures on the original songwriter agreements for their 1968 classic 'Do It Again', for example, or the original publisher's lead sheet for 'Help Me, Rhonda', and even the Beach Boys' own copy of the Library of Congress copyright certificate for 'Good Vibrations' – these take on a significance that the people who generated them could scarcely have dreamed of back in the early 1960s."

The Beach Boys themselves confirmed knowledge of the sale, but declined to comment further. It is understood they are unaware of how the vast archive of material came to end up in the storage unit, where it is thought to have languished since the early 1980s.

Ted Owen, the chief executive of Fame Bureau which is auctioning off the archive, said: "The finding of this huge archive is probably one of the great rescue stories in contemporary music history. This written record of the Beach Boys' creativity represents the largest and most valuable collection of its kind ever to reach an auction room."

The collection will be previewed in New York on 15 April and at the Hard Rock Café in London on 18 April