In the earlier versions of The Bible App, YouVersion adopted a three letter code for the books of the Bible. Here is a listing of those codes:
Gen = Genesis
Exo = Exodus
Lev = Leviticus
Num = Numbers
Deu = Deuteronomy
Jos = Joshua
Jdg = Judges
Rut = Ruth
Sa = Samuel for multi named books, leave off the number
Ki = Kings the number precedes to make it a 3 digit code (e.g. 1 Kings = 1Ki)
Ch = Chronicles
Ezr = Ezra
Neh = Nehemiah
Est = Esther
Job = Job
Psa = Psalms
Pro = Proverbs
Ecc = Ecclesiastes
Sng = Song of Solomon
Isa = Isaiah
Jer = Jeremiah
Lam = Lamentations
Ezk = Ezekiel
Dan = Daniel
Hos = Hosea
Jol = Joel
Amo = Amos
Oba = Obadiah
Jon = Jonah
Mic = Micah
Nam = Nahum
Hab = Habakkuk
Zep = Zephaniah
Hag = Haggai
Zec = Zechariah
Mal = Malachi
Mat = Matthew
Mrk = Mark
Luk = Luke
Jhn = John
Act = Acts
Rom = Romans
Co = Corinthians
Gal = Galatians
Eph = Ephesians
Php = Philippians
Col = Colossians
Th = Thessalonians
Ti = Timothy
Tit = Titus
Phm = Philemon
Heb = Hebrews
Jas = James
Jn = John
Pe = Peter
Jud = Jude
Rev = Revelations