I don't know what types of books are missing from these corpuses (corpii? :) ) though.
The plural of corpus in English is corpora, in common use by practitioners of corpus linguistics. Same thing in Latin, more or less: the nominative singular is corpus, nominative plural corpora. It's neuter third declension; the plurals that end in -i are typically masculine, second declension. But I'm afraid we're getting both off-topic and pedantic.