While Mated to the Warriors is the second book by publication, the series has several "sub-series" that also have their own second volumes:
Book Spotlight: Mated to the Warriors (Interstellar Brides® Program, Book 2) Grace Goodwin Interstellare BrГ¤ute Programm 2...
A recurring theme in the series where human females are often matched with multiple alien warriors to balance the warriors' intense needs. While Mated to the Warriors is the second