Darkening Skies by Juliet E McKenna

"In Halferan, Captain Corrain is hailed as a hero; but he knows all praise would turn to anger if the Caladhrians knew the truth. The wizard who supposedly saved them has merely claimed the corsair island for his own, and no one knows what his next move will be. Corrain has good reason to fear the worst, as he confides in Lady Zurenne. He knows he can trust her, now that still more perilous secrets bind them together. This disastrous turn of events cannot be concealed from Hadrumal's mages. Surely the Archmage will enforce his authority by crushing this upstart mage? Unless the Aldabreshin warlords act first. Will they be content to drive this solitary wizard out of the Archipelago, or has the time come for them to destroy all magic?"

Mass market, ex-library with usual markings