Previous IQs:
Were you the cranky lawyer in Heinlein’s Stranger in a Strange Land? - Jubal Harshaw
Was your son employed by Frederick the Great, whom you once met? - J.S. Bach, whose son C.P.E. Bach worked for the Prussian king; Bach the elder and the king even performed some chamber music together during his visit.
Our next letter is
V
And an update: Botticelli - the letters we've used so far - #67 by Elendil_s_Heir