bibfetch: ask every register that answers about a book, in turn

An ISBN is not a DOI. The body that assigns a DOI is the only one that has
it, so one register is the whole answer. A book's identifier is assigned by
its publisher and recorded by whoever gets round to it, and the record that
comes back is as often as not missing the thing the caller wanted: asked
about 9780199205592, OpenLibrary's edition record names *The First World
War* and the press that printed it, and nobody at all as its author, because
an edition record names its authors by key.

So an ISBN now has a chain rather than a register. The edition record first,
then OpenLibrary's own index — which answers with `author_name` filled in
and costs nothing — and Google Books last, because it answers anonymous
callers only until a daily quota runs out, after which it replies 429 to
everyone behind the same address, as it is doing here today.

The first complete answer wins, an incomplete one is kept in case nothing
better comes, and a register being down is a failure only when every
register is. `source` on the record says which one answered.

colophon comes with it: the imprint reader was taking `Oxford is a
registered trade mark of Oxford University Press` for a publisher and
`Oxford N`, off `Oxford New York`, for a place. A legal notice mentioning a
press is not the press, and a city is the city and not the first letter of
the next one.

crates/bibfetch/README.md

@@ -84,7 +84,20 @@ is a failure — nobody replied, and the identifier may be perfectly good.
 |---|---|---|
 | `doi` | Crossref | It assigns most scholarly DOIs |
 | `arxiv` | the arXiv | Its own register of preprints |
-| `isbn` | OpenLibrary | Books, without a key |
+| `isbn` | OpenLibrary's edition record, then its index, then Google Books | Books, without a key |
+
+One register is enough for a DOI or a preprint: the body that assigns the
+identifier is the only one that has it. A book is different. An ISBN is
+assigned by a publisher and recorded by whoever gets round to it, so an
+answer that names the book but not who wrote it is common — OpenLibrary's
+edition record names its authors by key rather than by name, and its index
+names them. So the registers are asked in turn and the first complete answer
+wins. An incomplete one is kept in case nothing better comes, and a register
+being down is a failure only when every register is.
+
+Google Books is asked last because it answers anonymous callers only until a
+daily quota runs out, after which it replies `429` to everyone sharing the
+address.
 
 An **ISSN** has no register that answers without a key, so it is not looked up
 on its own. A journal's ISSN comes back on the works published in it instead.