I’m after a book about an amphibian race, a guy found a creature, there was a scientist involved, the thing learnt English and lives in an advanced civilisation under the Earth that can be accessed through a cave under the sea. Something about peanut butter and jelly sandwiches and down there you press a button on a machine that makes a pill that tastes like the food you wish for or something like that my memory is fuzzy I read this cover less book when I was 9 (it was like .50c from a thrift store) and I think it was 60’s or 70’s maybe earlier again not sure.
Apparently Stranger from the Depths, by Gerry Turner, which has been identified here in the past, by "John S" back in 2014:
Found the answer on What's That Book.
Descriptions of the book:
"After sleeping for thousands of years the last survivor of a lost civilization returns to life to reveal secrets of the earth's core never imagined."
"a fantastic tale of two brothers who encounter strange beings who live below the ocean floor."
"It came out in the early 70's and was about the discovery of a coffin shaped stasis box buried under the ocean containing the last surviving member of an intelligent amphibious race..." "...a green man with webbed hands and feet"
"...underwater cities of Haad and Gaan"
"...A statue of one of the reptillian people. One of the reptillians separated from his people (maybe because of suspended animation?), ... The book started off with the protagonists setting up to photograph a tidal wave, with the wave washing away the rock exposing the diamond statue. The lizard's name was 'Saa', he liked peanut butter and jelly sandwiches..."
"... first Gary and his older brother filmed a tsunami, after which they came across a small statue of a mysterious ... Then Gary and his ... girlfriend Lori 'fess up about the underwater cave they've been sneaking away to, leading adults to organize a diving expedition to see what's down there. They find a large cavern containing a suspended animation capsule that contains a dead ringer for their statue. After they manage to revive him, their new friend Saa leads them down to his abandoned city underwater."
While not fitting your full description, Invasion from Below the Earth by Philip Curtis sounds somewhat like this. To address each of your details:
A guy found a creature
The box was so clear, it seemed to be made of glass. And inside it, sitting at some kind of control center, was a creature.
An amphibian race
The race is not quite amphibian, but:
The creature itself looked harmless, but very unhealthy. Its ale skin showed the insides of its body. It reminded Selwyn of a crab he had once caught – though the shape of the creature looked much more like a human than a crab.
Water poured out of his fishy eyes as if shot from a leaky water pistol.
There was a scientist involved
No human scientists are mentioned, but the Burrowers have scientists:
It contained the most modern equipment and two scientific Burrowers.
The thing learnt English
They have translating machines that enable them to speak English:
One of the Burrowers saw that Selwyn was trying to talk to him. He turned the translating machine on.
And lives in an advanced civilisation under the Earth
So the First Burrower, that great explorer to whom we Burrowers all owe our lives, decided to leave the surface world and spend the rest of his days with his friends under the ground. The more seaweed they ate, the smarter they became. Soon they were able to use the rocks, the water, and many metals for food, homes, and anything else the Burrowers needed."
That can be accessed through a cave under the sea
"Ages ago," explained the sub-Burrower, "some of the smartest members of a tribe in America found an underground stream. It flowed down deep into the earth. They could find no food to keep them alive on the surface. For many years the winters had become longer and colder. an ice age was coming, so they had nothing to lose. They hoped the underground climate would be warmer. They followed the stream down for many days. Just as they thought they would starve to death, they found a wonderful cave next to a huge lake.
Although the great underground cave was heated, they were all shaking.
Something about peanut butter and jelly sandwiches
There's nothing about sandwiches.
And down there you press a button on a machine that makes a pill that tastes like the food you wish for or something like that my memory is fuzzy
There is nothing about this either.
I read this cover less book when I was 9 (it was like .50c from a thrift store) and I think it was 60’s or 70’s maybe earlier again not sure.
The book was published in 1981, and can be bought at a very low price.