For those of us not fortunate enough to have day-tripped through the Summer of Love, psychedelia will always remain a mystery. We know it only through the Day-Glo swirl of au naturel concert pics, collectible OZ covers, and fevered rereadings of “The Electric Kool-Aid Acid Test.” Musically, “Sgt. Pepper’s” or “Electric Ladyland” are as deep as most dare to wade. But for an immersion experience, nothing beats Britain’s Incredible String Band. “The Circle Is Unbroken,” a new twodisc set of ISB demos and live recordings issued this week by Sanctuary Records, offers just such an opportunity.

Like all treasured artifacts of a former age, it’s easy to exaggerate the Incredible String Band’s prominence. The liner notes manage it with a few quotations. NME called “5000 Spirits or the Layers of the Onion,” the band’s 1967 album, “virtually the Bible of the burgeoning acid generation,” Robert Plant saw the band as “an inspiration and a sign,” and Paul McCartney apparently once remarked that “5000 Spirits” was his favorite album of 1967. It would be more accurate to remember that the band only briefly cracked the U.K. charts in 1969, and played a forgettable (and now forgotten) set at Woodstock.

For the new generation just discovering the band, however, their marginality is an asset. They’ve become a hipster shibboleth. Pavement’s Stephen Malkmus rekindled interest in ISB in 2000 by recklessly declaring them “the greatest band of all time.” In an arcanacred masterstroke, Brooklyn psych-rockers Oneida named a song after their girlfriends, “Rose and Licorice.” More recently, the New Weird America has discovered them through their association with Vashti Bunyan, the even more obscure late ’60s folk singer whose career has been resurrected by Devendra Banhart and company. (ISB’s Robin Williamson played on Bunyan’s excellent “Just Another Diamond Day.”)

Interestingly, the Incredible String Band’s self-titled 1966 debut belongs more to Newport than Haight-Ashbury. The cover shows Mike Heron, Williamson, and the soon to depart Clive Palmer looking like clean-cut folkies holding stringed instruments of all shapes and sizes. Their displays of multi-instrumental proficiency and emphasis on American bluegrass and Celtic folk traditions make them sound like a looser New Lost City Ramblers. There’s a hint of things to come, however, in the stoner metaphysics of “Smoke Shoveling Song”: “a thousand-foot-high way into the sky / was a pillar of smoke full of song / there was an airplane stuck in it, but I didn’t notice at first / it was so cunningly disguised as a dragon.”

After briefly scattering to the winds (Williamson studied music in Morocco, Heron traveled to Afghanistan), the duo got back together for their 1967 psych-folk classic, “5000 Spirits or the Layers of the Onion.” The change in look and outlook is a measure of how rapidly the scene transmogrified in the late 1960s. “It was nice for us, because we’d come out of a purist folk scene, which was a bit restricting,” Heron explains in the liner notes. “Suddenly to be invited into the psychedelic culture – a whole new era with different kinds of things – was very exciting.”

Although they remained strictly acoustic, unlike so many of their peers, their sound was undeniably psychedelic, incorporating a grab-bag of instruments Williamson brought back from North Africa (tambouras, sitar, gimbri, oud). Disc 1 of “The Circle Is Unbroken,” taken mostly from a demo session in Chelsea in 1967, offers an early look at this new direction and material.

In some respects, it’s a strange document – an obscurities collection for what is already an obscurity. Though that will suit their new fan base just fine. You can almost hear the conversations taking place all over Williamsburg and the Lower East Side: “Yeah, but have you heard the outtake version of ‘Little Cloud’?” the suede-vest wearing art-school grad asks the pretty-faced girl in the peasant dress as they sip their cardamom coffee. “It’s heavy, man. So heavy.”

There are rougher, folkier versions of some favorites from “5000 Spirits.” Especially good is a sprightly rendition (more mandolin, no bass) of “First Girl I Loved,” the bittersweet note-in-a-bottle Williamson wrote to a teenage sweetheart he fondly remembered while “rushing around Britain with a guitar making love to people that I didn’t even like to see.”

There’s also a lot of material that never made the album, some as good as anything on it. “Lover Man,” a kind of naive hill-country blues, opens with the charming line, “Baby, come tell me about your tree house / and your candy-striped pet mouse / and your car that has feet / hey, come tell me about your eyebrows that meet.” “Alice Is a Long Time Gone,” a companion to the album track “The Mad Hatter’s Song,” is told from the vantage point of a nostalgic White Rabbit. And “God Dog” is a whimsical, organ-driven number about a pet with “eyes as fine as the music of flutes” that “walks on the water without any boots.” (It was eventually recorded by Shirley and Dolly Collins for their 1969 album “Anthems in Eden.”)

By the early 1970s, the Incredible String Band was already an anachronism. They tried, flailingly, to keep up with the times by converting to Scientology, going electric, and getting into mime and dance. The sorry result can be heard on disc 2 of this set. Culled from a 1972 concert in Canada, it swerves between styles, each worse than the next. There’s middling Band-inspired folk rock (“I Know That Man”), throw-away Irish jigs and Joplin rags, and one song (“Old Buccaneer”) on which they sound like the Velvet Underground’s drunk uncles.

It will be of little interest to listeners brought in by “5000 Spirits,” and may even scare some of them off. Proof that, like the era that spawned them, the Incredible String Band was too good – and too weird – to last.