A book on the themes in northern European -- British, German, Scandinavian -- folklore, with some bits roving farther afield on some themes.
Each chapter has a theme -- passage of time in Fairy, slower or (less typically) faster -- the fairies' moral views (and double standards) on lying and stealing and other things -- types of fairies -- fairies' human captives -- and more. Some themes weave through it, like the ambiguous relationship between the fairies and the dead. (not just that apparently dead people might really have been taken; fairies were really and truly dead.)
I liked the examples given. Some of which were new to me even with some familiarity with the folklore. The first "King of Cats" tale recorded was of a troll disguised as a cat to hide from an unreasonably jealous husband. Some fairies can turn into doves but shrink a little every time they return to their native form. One woman told her husband how to rescue her from the fairies; it involved milk from a certain cow, unwatered. Alas, he had remarried, and his new wife watered it, and the fairies murdered the first wife. Mermaids have a knack for herbs and healing. One fairy bride lived with her husband only as long as he did not mention death to her.