My first computers were the IBM 1620 & CDC 1604; first languages: Fortran II, SPS assembly, CDC CODAP assembler. I also had a Hallicrafters SSB receiver, and a Heathkit 80 meter (or was it 40m?) transmitter. You guys are making me feel old
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Back on topic: it's best to think of the files used by Calibre to store ebook content as records in a black box database.