Simple AWS IAM Group policy to limit a client to read-only access to a single bucket. They'll be able to see the names of all other buckets, but won't be able to get into them. They will be able to see all folders and files in the bucket.

read_one_bucket.json

{
  "Statement": [
    {
      "Effect": "Allow",
      "Action": ["s3:ListBucket", "s3:ListAllMyBuckets" ],
      "Resource": "arn:aws:s3:::*"
    },
   {
      "Effect": "Deny",
      "Action": ["s3:ListBucket"],
      "NotResource":["arn:aws:s3:::bucketname", "arn:aws:s3:::bucketname/*"]
    },
    {
      "Effect": "Allow",
      "Action": ["s3:ListBucket","s3:GetObject"],
      "Resource": ["arn:aws:s3:::bucketname", "arn:aws:s3:::bucketname/*"],
      "Condition": {}
    }
  ]
}