A children's comic fantasy novel by Tanith Lee.
This is one of the first of Tanith Lee's books to be published, and is quite different from the dark fantasy that she is best known for. In tone it's close to Patricia C. Wrede's Dealing with Dragons series. It is very much a children's book; I would estimate that it would probably be best for ages 6-12 (plus anyone else who likes incompetent evil villains, talking animals, and daft kings). The writing style is not overly simplistic, although it is an easy read. There is a lot of deus ex machina (not necessarily a major sin in a comedy) and general silliness (ditto).
There's not a lot to tell about the story; the royal twins, Goodness and Jasleth, are having a large feast to celebrate their 17th birthday. Naturally, they've forgotten to invite an certain evil enchantress that they happen to be related to; she shows up anyway, and puts evil spells on them both (Jasleth will turn into a raven for a random hour each day, and Goodness will be so good it's silly). Goodness quickly begins giving away her parents' wealth to the local beggars, and Jasleth is sent on a quest to 'seek his fortune' (to find more wealth? to break the spell? we aren't told...) He eventually hooks up with Prince Fearless and crew, who are after the Dragon Hoard. They have any number of adventures involving evil wizards, merfolk, princesses, and dangerous tasks.
Mild Spoilers By the end of the story the curses are lifted, the enchantress punished, the Hoard captured, and all the princes and princesses have found their true loves.
It was originally published in 1971 (Farrar, Straus and Giroux). The 1984 (Tempo Books) reprint I have claims to be number 6 of the MagicQuest series. MagicQuest seems to be a completely unrelated group of fantasy books, all by different authors with very different writing styles. ISBN 0-441-16621-0.