I love that Soundtrack too!Aereo-plain was a great album. And he continued to write great songs, but I lost touch with him by the mid-70's. Last time I saw him was in April 1974 at a bluegrass festival in Marin County.
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I will suggest something else, just as good that you Bluegrass lovers can check out.
Seek out The Dillards, starting with 1968's Wheatstraw Suite. It opens with another Albert E. Brumley number (the same guy that wrote "Turn Your Radio On"), "I'll Fly Away." It has the first version of "The Hey Boys" - later done even better on Herb Pedersen's first solo, Southwest (hard to find, but worth the price of the Japanese Import). I am a major Herb fan. He did my favorite Beatle cover ever - "Paperback Writer" on that same long-player.
The Dillards' had a way with Beatle covers too. On Wheatstraw they chose "I've Just Seen A Face." Also here is "She Sang Hymns Out of Tune." If you can't find or afford the Import that puts together Pickin' and Fiddlin', Wheatstraw Suite, and Copperfields - a UK Electra two-disc set, that also includes their Capitol singles all in one neat set, some of them were released on Collector's Choice in the States. But try for the Import first, it's mastered much better.
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Copperfields (1970) was Herb's second album with the Dillards (he replaced Doug, who is on the Pickin' album from 1965). The Great John Boylan produced (Wiki that name). Herb and the boys wrote most of the tracks, but did cover "Yesterday" "Rainmaker" (lots of versions of that one around), and Eric Andersen's "Close The Door Lightly."
I haven't checked the prices these days, and I know as Brother Woody says, they don't stay in print long...But if nothing else, I bet you could find the old Vanguard Records 1991 comp that high-lights their out-put from 1963-1970 - There Is A Time, that includes a Dylan number, ""Walkin' Down The Line," before he was well known. And yes, that was The Dillards on the old Andy Griffiith Show in the early 60's. That's were everybody first heard them.
Herb left the band in 1970, but would come back now and then to play on later albums. I never got to see Herb all that many times over the years, but the first time was at that very Marin festival in 1974 - he was in Emmylou's Band, this was before she even had a record out. John did a set with his band, and came back for the festival closer Nitty Gritty Dirt Band. Another picker trying to make a name for himself was there as well, some cat called Steve Martin - quite popular these days.
I did a quick check of what else they had by John (thanks for the links) on Youtube, and found a great live take on "Piece of My Heart' from Cotati Caberet 04/21/83.