From the longer Wikipedia article [1]

Jan Marek Marci (13 June 1595 – 10 April 1667), or Johannes (Greek: Ioannes) Marcus Marci, was a Bohemian doctor and scientist, rector of the University of Prague [2], and official physician to the Holy Roman Emperors (Ferdinand III [3] and Leopold I [4]). The crater Marci on the far side of the Moon is named after him.

Marci at some time came into possession of the Voynich Manuscript, apparently upon the death of its former owner, the alchemist Georg Baresch. He sent the book to his longtime friend Athanasius Kircher, with a cover letter dated 19 August 1666, or possibly 1665.

More details can be found on the Wikipedia page [5] the Voynich Manuscript Biographies page [6], here [7] and on the Galileo Project [8]. Project Gutenberg has a text at [9].

A link to an image of the letter, now at the Beinecke Rare Book and Manuscript Library, can be found here [10].