For a user-friendly list of three letter abbreviations to Bible books that’s used by the Bible widget, see https://hackathon.bible/book-abbreviations-for-bible-widget/
The following is JSON output from https://api.scripture.api.bible/v1/bibles/bba9f40183526463-01/books (with an active API key)—
{
"data": [
{
"id": "GEN",
"bibleId": "bba9f40183526463-01",
"abbreviation": "GEN",
"name": "Genesis",
"nameLong": "Genesis"
},
{
"id": "EXO",
"bibleId": "bba9f40183526463-01",
"abbreviation": "EXO",
"name": "Exodus",
"nameLong": "Exodus"
},
{
"id": "LEV",
"bibleId": "bba9f40183526463-01",
"abbreviation": "LEV",
"name": "Leviticus",
"nameLong": "Leviticus"
},
{
"id": "NUM",
"bibleId": "bba9f40183526463-01",
"abbreviation": "NUM",
"name": "Numbers",
"nameLong": "Numbers"
},
{
"id": "DEU",
"bibleId": "bba9f40183526463-01",
"abbreviation": "DEU",
"name": "Deuteronomy",
"nameLong": "Deuteronomy"
},
{
"id": "JOS",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JOS",
"name": "Joshua",
"nameLong": "Joshua"
},
{
"id": "JDG",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JDG",
"name": "Judges",
"nameLong": "Judges"
},
{
"id": "RUT",
"bibleId": "bba9f40183526463-01",
"abbreviation": "RUT",
"name": "Ruth",
"nameLong": "Ruth"
},
{
"id": "1SA",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1SA",
"name": "1 Samuel",
"nameLong": "1 Samuel"
},
{
"id": "2SA",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2SA",
"name": "2 Samuel",
"nameLong": "2 Samuel"
},
{
"id": "1KI",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1KI",
"name": "1 Kings",
"nameLong": "1 Kings"
},
{
"id": "2KI",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2KI",
"name": "2 Kings",
"nameLong": "2 Kings"
},
{
"id": "1CH",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1CH",
"name": "1 Chronicles",
"nameLong": "1 Chronicles"
},
{
"id": "2CH",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2CH",
"name": "2 Chronicles",
"nameLong": "2 Chronicles"
},
{
"id": "EZR",
"bibleId": "bba9f40183526463-01",
"abbreviation": "EZR",
"name": "Ezra",
"nameLong": "Ezra"
},
{
"id": "NEH",
"bibleId": "bba9f40183526463-01",
"abbreviation": "NEH",
"name": "Nehemiah",
"nameLong": "Nehemiah"
},
{
"id": "EST",
"bibleId": "bba9f40183526463-01",
"abbreviation": "EST",
"name": "Esther",
"nameLong": "Esther"
},
{
"id": "JOB",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JOB",
"name": "Job",
"nameLong": "Job"
},
{
"id": "PSA",
"bibleId": "bba9f40183526463-01",
"abbreviation": "PSA",
"name": "Psalms",
"nameLong": "Psalms"
},
{
"id": "PRO",
"bibleId": "bba9f40183526463-01",
"abbreviation": "PRO",
"name": "Proverbs",
"nameLong": "Proverbs"
},
{
"id": "ECC",
"bibleId": "bba9f40183526463-01",
"abbreviation": "ECC",
"name": "Ecclesiastes",
"nameLong": "Ecclesiastes"
},
{
"id": "SNG",
"bibleId": "bba9f40183526463-01",
"abbreviation": "SNG",
"name": "Song",
"nameLong": "Song"
},
{
"id": "ISA",
"bibleId": "bba9f40183526463-01",
"abbreviation": "ISA",
"name": "Isaiah",
"nameLong": "Isaiah"
},
{
"id": "JER",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JER",
"name": "Jeremiah",
"nameLong": "Jeremiah"
},
{
"id": "LAM",
"bibleId": "bba9f40183526463-01",
"abbreviation": "LAM",
"name": "Lamentations",
"nameLong": "Lamentations"
},
{
"id": "EZK",
"bibleId": "bba9f40183526463-01",
"abbreviation": "EZK",
"name": "Ezekiel",
"nameLong": "Ezekiel"
},
{
"id": "DAN",
"bibleId": "bba9f40183526463-01",
"abbreviation": "DAN",
"name": "Daniel",
"nameLong": "Daniel"
},
{
"id": "HOS",
"bibleId": "bba9f40183526463-01",
"abbreviation": "HOS",
"name": "Hosea",
"nameLong": "Hosea"
},
{
"id": "JOL",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JOL",
"name": "Joel",
"nameLong": "Joel"
},
{
"id": "AMO",
"bibleId": "bba9f40183526463-01",
"abbreviation": "AMO",
"name": "Amos",
"nameLong": "Amos"
},
{
"id": "OBA",
"bibleId": "bba9f40183526463-01",
"abbreviation": "OBA",
"name": "Obadiah",
"nameLong": "Obadiah"
},
{
"id": "JON",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JON",
"name": "Jonah",
"nameLong": "Jonah"
},
{
"id": "MIC",
"bibleId": "bba9f40183526463-01",
"abbreviation": "MIC",
"name": "Micah",
"nameLong": "Micah"
},
{
"id": "NAM",
"bibleId": "bba9f40183526463-01",
"abbreviation": "NAM",
"name": "Nahum",
"nameLong": "Nahum"
},
{
"id": "HAB",
"bibleId": "bba9f40183526463-01",
"abbreviation": "HAB",
"name": "Habakkuk",
"nameLong": "Habakkuk"
},
{
"id": "ZEP",
"bibleId": "bba9f40183526463-01",
"abbreviation": "ZEP",
"name": "Zephaniah",
"nameLong": "Zephaniah"
},
{
"id": "HAG",
"bibleId": "bba9f40183526463-01",
"abbreviation": "HAG",
"name": "Haggai",
"nameLong": "Haggai"
},
{
"id": "ZEC",
"bibleId": "bba9f40183526463-01",
"abbreviation": "ZEC",
"name": "Zechariah",
"nameLong": "Zechariah"
},
{
"id": "MAL",
"bibleId": "bba9f40183526463-01",
"abbreviation": "MAL",
"name": "Malachi",
"nameLong": "Malachi"
},
{
"id": "MAT",
"bibleId": "bba9f40183526463-01",
"abbreviation": "MAT",
"name": "Matthew",
"nameLong": "Matthew"
},
{
"id": "MRK",
"bibleId": "bba9f40183526463-01",
"abbreviation": "MRK",
"name": "Mark",
"nameLong": "Mark"
},
{
"id": "LUK",
"bibleId": "bba9f40183526463-01",
"abbreviation": "LUK",
"name": "Luke",
"nameLong": "Luke"
},
{
"id": "JHN",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JHN",
"name": "John",
"nameLong": "John"
},
{
"id": "ACT",
"bibleId": "bba9f40183526463-01",
"abbreviation": "ACT",
"name": "Acts",
"nameLong": "Acts"
},
{
"id": "ROM",
"bibleId": "bba9f40183526463-01",
"abbreviation": "ROM",
"name": "Romans",
"nameLong": "Romans"
},
{
"id": "1CO",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1CO",
"name": "1 Corinthians",
"nameLong": "1 Corinthians"
},
{
"id": "2CO",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2CO",
"name": "2 Corinthians",
"nameLong": "2 Corinthians"
},
{
"id": "GAL",
"bibleId": "bba9f40183526463-01",
"abbreviation": "GAL",
"name": "Galatians",
"nameLong": "Galatians"
},
{
"id": "EPH",
"bibleId": "bba9f40183526463-01",
"abbreviation": "EPH",
"name": "Ephesians",
"nameLong": "Ephesians"
},
{
"id": "PHP",
"bibleId": "bba9f40183526463-01",
"abbreviation": "PHP",
"name": "Philippians",
"nameLong": "Philippians"
},
{
"id": "COL",
"bibleId": "bba9f40183526463-01",
"abbreviation": "COL",
"name": "Colossians",
"nameLong": "Colossians"
},
{
"id": "1TH",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1TH",
"name": "1 Thessalonians",
"nameLong": "1 Thessalonians"
},
{
"id": "2TH",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2TH",
"name": "2 Thessalonians",
"nameLong": "2 Thessalonians"
},
{
"id": "1TI",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1TI",
"name": "1 Timothy",
"nameLong": "1 Timothy"
},
{
"id": "2TI",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2TI",
"name": "2 Timothy",
"nameLong": "2 Timothy"
},
{
"id": "TIT",
"bibleId": "bba9f40183526463-01",
"abbreviation": "TIT",
"name": "Titus",
"nameLong": "Titus"
},
{
"id": "PHM",
"bibleId": "bba9f40183526463-01",
"abbreviation": "PHM",
"name": "Philemon",
"nameLong": "Philemon"
},
{
"id": "HEB",
"bibleId": "bba9f40183526463-01",
"abbreviation": "HEB",
"name": "Hebrews",
"nameLong": "Hebrews"
},
{
"id": "JAS",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JAS",
"name": "James",
"nameLong": "James"
},
{
"id": "1PE",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1PE",
"name": "1 Peter",
"nameLong": "1 Peter"
},
{
"id": "2PE",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2PE",
"name": "2 Peter",
"nameLong": "2 Peter"
},
{
"id": "1JN",
"bibleId": "bba9f40183526463-01",
"abbreviation": "1JN",
"name": "1 John",
"nameLong": "1 John"
},
{
"id": "2JN",
"bibleId": "bba9f40183526463-01",
"abbreviation": "2JN",
"name": "2 John",
"nameLong": "2 John"
},
{
"id": "3JN",
"bibleId": "bba9f40183526463-01",
"abbreviation": "3JN",
"name": "3 John",
"nameLong": "3 John"
},
{
"id": "JUD",
"bibleId": "bba9f40183526463-01",
"abbreviation": "JUD",
"name": "Jude",
"nameLong": "Jude"
},
{
"id": "REV",
"bibleId": "bba9f40183526463-01",
"abbreviation": "REV",
"name": "Revelation",
"nameLong": "Revelation"
}
]
}