@hradocan: You are overcomplicating it. If you look at how most of the metadata source plugins handle this, the "create_query" will check if there is an ISBN in the identifier and build a query that just uses that. If not, builds the author/title query. Then it does the actual search. If nothing is found, they recurse into "identify" again without passing in the identifiers.
You can look at the Goodreads, KoboBooks and others to see how they handle this.
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