Project Gutenberg identifiers

Archive-reader recognizes ProjectGutenberg as a known identifier scheme when a publication states it explicitly. A publication can also carry a Project Gutenberg reference as a general URL identifier.

Publication metadata	Resolved identifier
EPUB 3 identifier explicitly typed ProjectGutenberg	{ value: id, scheme: "ProjectGutenberg" }
Absolute Gutenberg URL in an EPUB identifier	{ value: url, scheme: "URL" }
Absolute Gutenberg URL in ComicInfo Web	{ value: url, scheme: "URL" }
Untyped opaque Gutenberg ebook number	{ value: id, scheme: "Unknown" }

Explicitly typed EPUB identifier

EPUB 3 can state the identifier namespace directly:

<dc:identifier id="gutenberg-id">78139</dc:identifier>
<meta refines="#gutenberg-id" property="identifier-type">
  ProjectGutenberg
</meta>

Archive-reader preserves the value and canonicalizes the known scheme spelling:

{
  value: "78139",
  scheme: "ProjectGutenberg",
}

This is the most precise representation because the publication explicitly says what assigned the otherwise ambiguous number.

Reference URL

Publications can carry a catalog reference without assigning a specialized identifier scheme.

In EPUB metadata:

<dc:identifier>https://www.gutenberg.org/ebooks/78139</dc:identifier>

In a CBZ or CBR ComicInfo.xml, the standard Web field provides the equivalent representation:

<ComicInfo>
  <Web>https://www.gutenberg.org/ebooks/78139</Web>
</ComicInfo>

Both resolve to the literal URL:

{
  value: "https://www.gutenberg.org/ebooks/78139",
  scheme: "URL",
}

Archive-reader normalizes an absolute HTTP(S) value generically as URL and preserves the complete authored value. ComicInfo keeps it under metadata.comicInfo.web as well, and its Web field can contain multiple space-separated reference URLs.

Reading the ebook number

Because the same identifier arrives either explicitly typed or as a reference URL, read it with identifierValue rather than filtering on the scheme:

import { identifierValue } from "@prose-reader/archive-reader"

const id = identifierValue(metadata.identifiers, "ProjectGutenberg")
// "78139" | undefined

That answers for both representations above. Filtering on scheme === "ProjectGutenberg" misses every publication — comics especially — that stated the reference as a URL.

See Publication identifiers for the shared scheme resolution rules, custom schemes, URLs, and the Unknown fallback.