Technically this is the third book in a series, but like Discworld or Witch World, the series is really a setting in which a number of books take place.
All right, the magical horse Valadan, half wind by descent, reappears, but given the time span between this and other stories, it's unsurprising that even his motives are distinct to this book.
It opens, however, with a raging Queen Melcia, having killed her husband the king -- they both had kingdoms, and this was an arranged match to join them -- descending on his mistress and their daughter Savrin, ordering them both drowned, and raging she can not bear to see her husband's face in the girl. Her mother is drowned, but Savrin, thrust into the waters, turned into an otter.
The next chapter opens with Titch challenging a passing knight to a fight. He has his sword from his father, and his horse, though Gray is aging, but he has yet to win to being a knight like his father -- and the man has a splendid horse. He loses, and guides Gerein to an inn where he can stay. There, Gerein forbids him to ever take the iron bit out of the horse's mouth, and Titch learns Gerein is being pursued by men after an accidental death.
It goes on from there. It involves Queen Melcia being as jealous and possessive about her son as her husband, a wizard whose apprentice, Wren, has a falcon but no memory of her past before she was transformed into an otter (the wizard turned her back), a swan seeking its mate, a prince who hates being freed from an enchantment, the effects of iron on magic, a tournament, men who leave Titch to die in the cold, Wren's ability to cure tooth ache, and a trial by combat, along with much more in a tale full of adventure and twists.
(Savrin's backstory, BTW, comes out of Celtic legend.)