Both the previous books are stories about the end of the world, whether it be by a bang (BtMB) or a long drawn out whimper (AtWS). The Sea Thy Mistress is about a new beginning, and the tension comes from the vulnerability of the newborn world. At the end of All the Windwracked Stars Muire willing took up the role of Bearer of Burdens and brought life back to the world. But the Lady Heythe has ridden out of the first ending of the world into this new beginning. The world changed while she wasn't there but she only aims to finish the job she started in By the Mountain Bound.
Read the rest of the review on my blog.