I’ve been looking for a book, but have been unable to find it – I believe it’s because I can’t remember any sufficiently specific keywords, but it’s also possible that I made the whole thing up. 
Context : it’s an early chapter book (see: Secrets of Droon, Fairy Realm) I would have read this somewhere between 2003-2010, but probably 2005-2006. This is part of a series of books, and I believe it’s the last one – probably the third or fourth book in the series. 
Physical description: I think it’s about 200-300 pages long, and I had it probably in a paperback, which I almost certainly got from the library. I think the cover was light (maybe a white background?) with some bright colors, and I distinctly remember both the cover art and the illustrations having a sparse, low-fi, doodle-y style. One of the things I remember best is that all the books had maps in the front, which I think were in black and white – this particular map shows the mythical islands where most of the story takes place, and I learned the word “archipelago” while reading it. It’s possible that the author was from the U.K., based on the vibes of the place names. 
Plot description: It’s a fantasy series (all the characters come from a fairly generic, medieval-y world, definitely none come from our world through a portal or anything). It features a young, plucky heroine, who is now a seasoned adventurer (despite her humble origins in the beginning of the series). She might have some kind of professional title (possibly “Royal Adventurer” or something of that ilk) – I’m pretty sure the first book in the series involves her being sent (or sending herself) to slay a dragon, but instead befriending it. I think at some point, she’s illustrated with curly, light-colored hair, wearing a horned helmet and an oversized chainmail shirt (possibly found in the dragon’s hoard?). In this book, she is caught in a storm (while flying on the dragon? Or perhaps in a boat? Did she sail into it on purpose?) and becomes stranded on an island that seemed to be part of myth (diegetically- she had found a partially destroyed map with some clues on it, while searching for some MacGuffin, which involved some kind of powerful object or noble person who had disappeared under unusual circumstances). On this island, there are people who might be mages of some sort, who heal her injuries, fix up her mode of transportation, and give her soft robes to wear – kind of a Calypso/ spa kind of thing. I think these people are also tall and unusually beautiful in some way. They have some kind of communal ritual involving music/singing, which maybe also controls the weather (possibly maintaining the storm that the heroine was caught in initially). It’s clear that the island is way better than the place that the heroine comes from (delicious food, clear crystalline water, warm weather, birds singing, what-have-you) and the people want to protect it or something. The heroine is then able to explore the archipelago and finds a resolution to her initial quest/MacGuffin. The people on the island offer the heroine a permanent place in their luxurious hidden society (are they trying to hide something suspicious, or rewarding her valor? Maybe they’re just like that) but she feels an obligation to the people who sent her on her initial quest, and so returns home. After this, she feels that her adventuring days have come to an end, and retires in a hilly place with sheep and apples. If she has a dragon, it might live in her barn or otherwise nearby. The place might have a whimsical, Irish-sounding name, like “Winsey”.