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I think it mainly goes by publisher who still owns the rights. Thus, Pern books from Random House and Ship books from Baen are available, but the original Ship Who Sang was some other publisher IIRC who has not yet seen fit to e-release.
Same with MZB as DAW/Penguin are sluggish about putting out the older backlist (even mid-2000s titles for some series I read which have later volumes out).
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