By Sean Russell. A duology consisting of World Without End and Sea Without a Shore.
This duology takes place about 40 years after The River Into Darkness, although it was written first.
It seems to me this is the other side of the story: even though some of the individual lives from The River Into Darkness ended in tragedy, this pair of books is fairly effective at depicting magic as dangerous and better left unknown and unused. I think I may have finished it due to muleheadedness rather than a particularly compelling narrative, but it's still decently written, even if there is little magic. Some of the details (what the Countess knows, for example) don't seem to quite match up with the prequel, but all-in-all it meshes fairly well with the later work.
You want a summary? Tristam Flattery, whose great-uncle Erasmus (a central character in The River Into Darkness) was reputed to be a mage, is summoned to the capital to try to revive Kingsfoil, a plant which, while surrounded with incredible secrecy and miraculous healing powers, may have another purpose. (If you've read The River Into Darkness you know what it is.) When he fails to make any progress, he is sent on a voyage to the island in the tropics where the plant was originally found to try to find more. Unfortunately, there seems to be plans surrounding the voyage that involve more than just keeping the ancient King alive by procuring more of the plant's seed.
(Spoilers may follow.)
I'm not sure I would have chosen to read this if I hadn't found out it was part of the same story as The River Into Darkness. I am still not happy with the ending of that story: it seems possible that Anna Fielding (whose name is not even mentioned here) could have been persuaded to desist her efforts. It is also interesting that the visions of the future seen are not the same in both duologies: The River Into Darkness presents a vision of atomic holocaust, while Moontide and Magic Rise presents a slightly less drastic vision of the horrors of urbanization and the industrial age. One is left to wonder if the worse horror was invented by the mages for the purpose of encouraging cooperation with their purpose (the extinction of magic) or if it was merely left unrecorded, too horrible to say.