This book is a direct sequel to Shadow of the Lion, but it's not quite as ambitious in scope. Shadow had multiple viewpoint characters, in different parts of the world, engaging in different activities that all wove together into one big plot. In This Rough Magic, there are still several viewpoint characters, and they are acting somewhat independently, but most of the action takes place on the island of Corfu, and what the different groups of people are up to is much more closely connected into one story.
Through several nasty levels of intrigue on the part of the various villains, Corfu, a Venetion possession, is besieged by vastly superior forces, with the protagonists (most of the major surviving good guys from the first book, including Benito, Manfred, Francesca, Manfred's bodyguard Erik, Benito's love Maria and her husband, and churchman-mage Eneko Lopez) ending up on the island before the siege is established. Though vastly outnumbered, the good guys have superior strategy and good luck on their side. A series of harrowing battles and feats of derring-do ensue, with ultimately unsurprising results.
I only noticed one inconsistency with the first book: in my memory, Marco had to agree at the end of Shadow that he would never leave Venice for anything -- yet he ventures outside of Venice twice in This Rough Magic, with no really compelling reason beyond the convenience of the authors.
Overall, it was well enough written to keep me turning pages, and (with the one exception I noted) faithful enough to the first one to maintain the excitement. But the world was already built, and the new things we discover are much less of a feast for the sense of wonder than Shadow of the Lion was. And the plot fell victim to a lot of the "well, the good guys have to win" predictability that is found in all but the best fantasy novels exhibit. To be fair, its worst failing is probably just not being up to the expectation set by Shadow, which I tought was outstanding. Don't get me wrong; I enjoyed it and I'd call it a good book. It just wasn't a great one. I give it an 8 out of 10.