Funnily enough my Goodreads Covers plugin had a cache for lookup by title/author for books that did not have an isbn. But after writing all the code for that version of the metadata api I found I couldn't actually do title/author searches, only isbn ones
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Now that I can store Goodreads id I don't need a cache by anything else for my plugin - once you have hit a result the cache will key it by goodreads id and I can pull it back by that in future.