These are the novels in Harry Turtledove's Darkness series, published by Tor from 1999 to 2004.
“Into the Darkness (Tor, 1999) is the first book in the Darkness series, which depicts a war...Into the Darkness
“Darkness Descending (Tor, 2000) is the second book of the Darkness series. Algarve has been...Darkness Descending
“Through the Darkness (Tor, 2001) is the third volume of the Darkness series by Harry Turtledove...Through the Darkness
“Rulers of the Darkness (Tor, 2002) is the fourth book in the Darkness series by Harry...Rulers of the Darkness
“Jaws of Darkness (Tor, 2003) is the fifth and penultimate volume of Harry Turtledove's Darkness...Jaws of Darkness