Short answers, pulled from the story.
Hayward and Blanche Cirker opened their book business in 1941 from an apartment building called The Dover in Forest Hills South, Queens. They began by selling remaindered textbooks through the mail to a small group of customers.
The first official publication released by Dover Publications was Jahnke and Emde's Tables of Functions with Formulas and Curves. This specific volume became available because United States law voided its German copyright during World War II.
Most Dover reprints appear as photo facsimiles of the originals that retain the original pagination and typeset while sometimes adding a new introduction. The company makes these books available at significantly reduced costs compared to standard market prices.
Dover's foray into recordings was not as successful as its core business of book republication due to lower recording levels creating more noise and vulnerability to scratches. The company eventually abandoned this audio venture entirely.
Hayward Cirker died in 2000 at the age of 82. In that same year, Dover Publications was acquired by Courier Corporation.