My thoughts on this book have changed quite a bit since I first read it more than two years ago. Heir to Sevenwaters is a fun book, and on the first couple of reads I loved it and considered it just as good as the original Sevenwaters books, mostly because the romance is fantastic. But while this book has its strengths, compared to the original trilogy it's still a bit shallow (and things have only gotten worse with the latest installment, Seer of Sevenwaters).
Heir to Sevenwaters is much less dark than previous installments. The heroine goes on a quest to recover her missing baby brother, taking up the bulk of the book. Less is at stake than in the original books, and the villain is neither sadistic nor holding a murderous grudge against Sevenwaters (instead, he comes across as merely confused and megalomaniacal). Also, whereas previous heroines had to deal with most of their problems on their own and keep secrets from people they loved, Clodagh has help for most of the book and doesn't have one of the agonizing internal conflicts characteristic of the previous books. And the "quest" setup makes this one structurally different as well. Both the page count and the time period covered are much shorter than previous books.
I still think this is a decent book, with a fast-paced plot and endearing characters. Marillier's dialogue has gotten much crisper since the original trilogy, which is great, but I do feel that she's lost something with the shift in her writing style: it seems less lush, less poetic, the emotions more trite.
Honestly, at this point I'm not sure whether I would recommend this book. It's fun, and especially for younger readers it's probably worth the time (it's a quick read). But the more Sevenwaters-sequels I read, the more I doubt that the trilogy should have been continued. As for those who haven't tried Marillier's books before: while this book can be read as a standalone, Daughter of the Forest is still the ideal introduction to her work.