There are 4 books in the Star Wars: The Old Republic - Legends series. The newest book is Annihilation which came out in 1998.

There’s something out there:
a juggernaut of evil bearing down to crush the Republic—
unless one lone Jedi, shunned and reviled, can stop it.

Revan: hero, traitor, conqueror, villain, savior. A Jedi who left Coruscant to defeat Mandalorians—and returned a disciple of the dark side, bent on destroying the Republic.…
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3
Fatal Alliance

Book cover of Fatal Alliance
Long-standing foes must put aside their ancient grudges and deep-seated distrust to uncover and defeat a threat to the galaxy before it can escape its isolation—forcing Sith and Jedi to raise their lightsabers together.

ā€œWhen I look into you, I see no loyalty. . . . I sense only tangled…

4
Annihilation

Book cover of Annihilation
Republic agent Theron Shan and his Twi'lek compadre, Teff'ith must contend with a Sith Empire counter-attack against the Republic, spearheaded by the lethal apprentice of Darth Malgus. Satele Shan and Jace Malcolm co-star in what will be a fast-paced and tension fraught tale based on the award-winning video game from…