Here are the links to my previous posts about the Sevenwaters series -
Daughter of the Forest and
Son of the Shadows/Child of the Prophecy.

I was very pleased to realise, when I returned the last two books of the series, that I was able to borrow this one! The joy about reading books from the library is that it's totally guilt free as the extra reads don't join the toppling TBR pile. Actually, now I think about it, not totally guilt free as by reading books from the library, I'm not exactly making inroads into the pile. Hrmmm. More ethical thinking required!
Anyway.
Heir to Sevenwaters
focuses on Clodagh, daughter to Lady Aisling and Lord Sean who is the twin brother of Liadan, heroine of Son of the Shadows. Keeping up? Good...
It's now three years after the events of Child of the Prophey, and at the start of the book, having given birth to six daughters, Aisling is pregnant however this is cause for concern rather than celebration as she is well past the safe age for childbearing. During her difficult pregnancy, management of the large household falls to the very ordinary and practical Clodagh as her father is distracted by concern for his wife as well as tension between rival neighbouring clans. A heightened sense of responsibility for others means that after her baby brother is born, Clodagh adds looking after him to her list of tasks until one night he is stolen and a changeling that only she can see is left in his place...
This book is more fairytale-ish than the others as Clodagh enters the Otherworld in search of her brother and to confront the powerful fairy Lord Mac Dara who lives there. Reluctantly helping her in her quest is Cathal, who is a fearsome warriors trained by her cousin, who has a very prickly nature and a lack of self-belief. Unlike previous Sevenwater heroines, Clodagh doesn't have any special abilities that help her fulfil her task so it's only her courage, quick thinking, ability to make friends and determination that can help her to succeed.
This book has more emphasis on romance than the darker Child of the Prophecy did - which was a definite plus for me. The developing trust and affection between Clodagh and Cathal is quite sweet if slightly lacking in emotional intensity. Like the other books in this series, the importance of duty and family is a central theme but there's plenty of adventure and magic too. One thing I would comment on is that it felt a bit less mature than the other books in the trilogy - which is not necessarily a bad thing - but the tone of the book struck me as more like that of her delightful young adult book
Wildwood Dancing so perhaps writing for a younger audience has influenced Marillier's style in the years between writing those books and this one. I enjoyed the latest installment in the Sevenwaters saga all the same and want more!