Th Seafolk, as their name indicates, live in the seas and oceans of the Hidden World. They are featured in the fourth book, Thomas Trew and the Selkie's Curse, but also appear in the third book, Thomas Trew and the Klint-King's Gold.
There's all kinds of Seafolk. Good ones, bad ones, and in between! There are mermaids and Syrens(dangerous mermaids); selkies(seal-people) and fin-folk(shark-people), and all sorts of others. There are dolphins and whales and fish and all sorts of other creatures too, some of which can communicate with the humanoid Seafolk like selkies and mermaids and such. The Seafolk's world is vast and much of it is wild and frightening, like the wasteland of Mirkengrim. But there are also villages and the huge city of Oceanopolis, which is where Thomas and his friends go. Oceanopolis, which is ruled by the Sea Duchess, is very rich and full of amazing mansions, shops and Seafolk!
The selkies are the Seafolk Thomas gets to know best. They're talkative, lively, bright, quarrelsome sort of people who have a great gift, the gift of music. From them comes much of the music of the Hidden World; so when that music is threatened, a dark cloud descends on all of the Hidden World..
You can start meeting the Seafolk, especially the selkies, in the third book, Thomas Trew and the Klint-King's Gold, which is officially out in Britain today!