3rd book of 7 in the Star Trek: Gateways series and known as The Next Generation volume.
"With the reappearance of the Iconians, an ancient alien civilization, and their offer to sell the secrets of their advanced technology to the Federation, chaos erupts all over the galaxy and Captain Picard and his crew contend with feuding Klingons and Romulans and with the sinister truth about the Iconians' return."
Note: Garak plays a small but interesting part.