See also: the BBC and especially this lovingly detailed article in the Guardian (UK):
Like many other questing spirits who came to age in the mid-60s, he was inspired by taking LSD to create truly daring, other-wordly music - first for the original incarnation of Pink Floyd, then as a solo singer/songwriter - but the drug ended up fatally fracturing his psyche and turning him into a solitary recluse unable to function within the music industry and society in general. The story of his personal meltdown has been told and retold as a cautionary tale for indiscriminate druggies to the point where Barrett's status as rock's most illustrious casualty often threatens to outweigh his actual creative contributions to the form. This is not as it should be.Although I am a fan of Syd Barrett-era Pink Floyd, I think that arguably Syd's finest achievement was the album he recorded after mental illness (combined with daily LSD use) had already begun to take very firm hold of his life.
This record, The Madcap Laughs, was almost a Pink Floyd album anyway, as Syd had considerable assistance in the studio from Roger Waters and David Gilmour, and was hugely influential on two generations of musicians that followed.