jq syntax error extracting key with colon in its name

I have the following JSON (which can be retrieved from https://openlibrary.org/api/books?bibkeys=ISBN:039397281X&format=json) as input:

{
  "ISBN:039397281X": {
    "bib_key": "ISBN:039397281X",
    "preview": "borrow",
    "thumbnail_url": "https://covers.openlibrary.org/b/id/7890978-S.jpg",
    "preview_url": "https://archive.org/details/isbn_9780393972818",
    "info_url": "https://openlibrary.org/books/OL348852M/Don_Quijote"
  }
}

My goal is to extract the preview_url -- thus, for the input above, to get the output https://archive.org/details/isbn_9780393972818.

I'm currently using the following:

curl 'https://openlibrary.org/api/books?bibkeys=ISBN:039397281X&format=json' \
  | jq -r '.ISBN:039397281X' | jq -r '.preview_url'

However, this emits an error:

jq: error: syntax error, unexpected ':', expecting $end (Unix shell quoting issues?) at <top-level>, line 1:
.ISBN:039397281X     
jq: 1 compile error

How can it be resolved?