[PoP]

Originally Posted by jhowell

...
the file extensions are .mbs and .mbp1 (for MOBI), .azw3f and .azw3r (for KF8), and .yjf and .yjr (for KFX)
...

Thanks for sharing. Also found that .pds and .pdt (for PDF) have that signature and decode similarly.

Spoiler:

.pds

{
    "font.prefs": {
        "typeface": "_INVALID_,und:bookerly",
        "lineSp": -1,
        "size": -1,
        "align": -1,
        "insetTop": 28,
        "insetLeft": 28,
        "insetBottom": 0,
        "insetRight": 28,
        "unknown1": -1,
        "bold": -1,
        "userSideloadableFont": "",
        "customFontIndex": -1,
        "mobi7SystemFont": "_INVALID_,und:bookerly",
        "mobi7RestoreFont": false,
        "readingPresetSelected": ""
    },
    "sync_lpr": false,
    "annotation.cache.object": {
        "annotation.personal.highlight": [
            {
                "startPosition": "1 6 51 1 255 177 53 17",
                "endPosition": "1 48 280 1 305 345 79 21",
                "creationTime": "2019-08-13T16:05:05.820000",
                "lastModificationTime": "2019-08-13T16:05:05.820000",
                "template": "0\ufffc0"
            },
            {
                "startPosition": "1 67 361 1 123 445 63 17",
                "endPosition": "1 108 9 1 450 1049 13 21",
                "creationTime": "2019-08-13T16:06:03.643000",
                "lastModificationTime": "2019-08-13T16:06:03.643000",
                "template": "0\ufffc0"
            }
        ],
        "annotation.personal.note": [
            {
                "startPosition": "1 48 280 1 305 345 79 21",
                "endPosition": "1 48 280 1 305 345 79 21",
                "creationTime": "2019-08-13T16:05:46.309000",
                "lastModificationTime": "2019-08-13T16:05:46.309000",
                "template": "0\ufffc0",
                "note": "Ingr\u00e9dients"
            },
            {
                "startPosition": "1 108 9 1 450 1049 13 21",
                "endPosition": "1 108 9 1 450 1049 13 21",
                "creationTime": "2019-08-13T16:06:23.148000",
                "lastModificationTime": "2019-08-13T16:06:23.148000",
                "template": "0\ufffc0",
                "note": "Recette"
            }
        ],
        "annotation.personal.bookmark": [
            {
                "startPosition": "1 0 0 0",
                "endPosition": "1 0 0 0",
                "creationTime": "2019-08-13T16:01:53.130000",
                "lastModificationTime": "2019-08-13T16:01:53.130000",
                "template": "0\ufffc0"
            }
        ]
    },
    "language.store": {
        "language": "fr",
        "unknown1": 0
    },
    "ReaderMetrics": {
        "booklaunchedbefore": "true"
    }
}

.pdt

{
    "fpr": {
        "position": "80 0 0 0",
        "time": null,
        "timeZoneOffset": null,
        "country": "",
        "device": ""
    },
    "page.history.store": [],
    "lpr": {
        "position": "1 0 0 0",
        "time": "2019-08-13T16:06:55.068000"
    }
}