Caretakers — Frank Carey

IMaintaining a sanctuary for alien lifeforms can be difficult. Dealing with an alien ship bent on revenge is downright hard.

After Area 51 maintenance worker Lee Carstares is exposed to experimental nanobots, he is reassigned to Area 26--the Extraterrestrial Zoo, where he and his partner, Clare, a grey alien from Zeta Reticuli, keep track of the strange and wonderful fauna from planets outside of Earth space. Their routine is shattered by the arrival of two alien spacecraft, one bent on destroying the other. In a race against time, the two caretakers must help one of the ships get back into space without alerting their pursuers while keeping the existence of both ships secret from the population of Earth.

Caretakers is the first book of the Prehistory of the League of Planetary Systems where feeding the livestock takes on a whole new meaning.