Bounty Hunter, Joruus C'boath is in Heir To The Empire, Dark Force Rising, and The Last Command. To read more about his background read Heir To The Empire Sourcebook, Dark Force Rising Sourcebook and The Last Command Sourcebook. Those are the only books that I know of that Joruus C'boath is in. Try reading Dark Empire, Dark Empire II and Empire's End (Dark Empire III)and Dark Empire Sourcebook if you want to know more about the "new" Emperor Palpatine/Darth Sidious.
I have The Thrawn trilogy novels AND the sourcebooks. I HAVE READ THEM ALL. Yes, Joruus C'boath IS the "The Guardian", BUT nowhere does it say EXACTLY when he became "The Guardian" or when Jorus C'boath EXACTLY died. This leaves some interesting posibilities for Jorus and Joruus C'boath (possible movie appearances).
Read the Thrawn Trilogy Sourcebook or even better the individual sourcebooks if you can find them. It DOESN'T say that Thrawn killed Jorus C'boath. In Heir To The Empire, Thrawn was confering with Captain Gilad Pellaeon, (another favorite character of mine) he was talking about both Jorus C'boath and the clone Joruus. He SPECULATED that Joruus was created right before Jorus was killed. In one of the sourcebooks it mentions that dead body of Jorus was in the large stone coffin in the crypt of Mt. Tantiss. Read The Last Command to find out Joruus's reaction when he finds out he is a clone of Jorus.