Short answers, pulled from the story.
Luca Pacioli is called the father of accounting because he was the first person to publish a work on the double-entry system of bookkeeping on the European continent. His 1494 Summa de arithmetica described journals, ledgers, the trial balance, and year-end closing entries in a form that remained the standard accounting textbook internationally until the mid-16th century.
Pacioli and Leonardo da Vinci met in Milan in 1497 after Pacioli accepted an invitation from Duke Ludovico Sforza. The two lived together, Pacioli taught Leonardo mathematics, and Leonardo drew the illustrations of regular solids for Pacioli's Divina proportione. They were forced to flee Milan together in 1499 when Louis XII seized the city, and their paths appear to have finally separated around 1506.
In 1494, Pacioli published Summa de arithmetica, geometria, Proportioni et proportionalita in Venice. It was a synthesis of the mathematical knowledge of his era, containing the first printed work on algebra in a spoken vernacular and the first major published description of the double-entry accounting system.
De viribus quantitatis is a treatise on mathematics and magic that Pacioli wrote between 1496 and 1508. It contains the first reference to card tricks in print, as well as instructions on juggling and fire eating, and is the first work to note that Leonardo da Vinci was left-handed. It was never published in Pacioli's lifetime and sat in the archives of the University of Bologna until its rediscovery; an English translation appeared for the first time in 2007.
Yes. The 16th-century art historian Giorgio Vasari accused Pacioli of plagiarism for using Piero della Francesca's work without attribution. The second volume of Pacioli's Summa was a slightly reworked version of one of Piero's works, and the third volume of Divina proportione was an Italian translation of Piero's Latin book De quinque corporibus regularibus, neither credited to Piero.
Pacioli's unpublished treatise on chess, De ludo scachorum, was rediscovered in 2006 in the library of Count Guglielmo Coronini-Cronberg in Gorizia, which holds some 22,000 volumes. A facsimile edition was published in Pacioli's home town of Sansepolcro in 2008.