There are 4 books in the Mass Effect series. The newest book is Deception which came out in 2001.

1
Revelation

Book cover of Revelation
The thrilling prequel to the award-winning video game from BioWare

Every advanced society in the galaxy relies on the technology of the Protheans, an ancient species that vanished fifty thousand years ago. After discovering a cache of Prothean technology on Mars in 2148, humanity is spreading to the stars; the…
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2
Ascension

Book cover of Ascension
Location: Our galaxy, at a time set far in our future ...Every 50,000 years, a race of machines returns to harvest the galaxy's organic life forms. In the first MASS EFFECT game, it is the year 2183, and the machines are about to return once again. This book will be…

3
Retribution

Book cover of Retribution
Humanity has reached the stars, joining the vast galactic community of alien species. But beyond the fringes of explored space lurk the Reapers, a race of sentient starships bent on 'harvesting' the galaxy's organic species for their own dark purpose. The Illusive Man, leader of the pro-human black ops group…

4
Deception

Book cover of Deception
This novel continues the all-new, completely original story begun in the first three MASS EFFECT stories. The heroine, Gillian, was once the subject of horrifying scientific experiments, but now, after her rescue, she is beginning to master her amazing powers. But when she falls under the sway of a radical…