H.N. Turtletaub is a nom de plume used by Harry Turtledove for five historical novels. The name was first used for Justinian, and was chosen because of the way the big chain stores purchase books. If the book, which was in a different genre than most of Turtledove's books, didn't do well, its lack of sales could have directly affected his future sales.
As it happened, the book sold well enough that the publisher offered a contract which resulted in the series of novels about Sostratos and Menedemos, the "Hellenic Traders". The four books in that series are Over the Wine-Dark Sea, The Gryphon's Skull, The Sacred Land, and Owls to Athens.
Turteltaub and Turtledove are German and English names for the same species of bird.