Three Little Wishes by Paul Cornell

Kelly Castleton is the most detail-oriented, by-the-book contract lawyer you could ever wish to have on your side unless of course you happen to be an immortal fairy with three malicious wishes to grant! Of course the day Kelly decides to be spontaneous on One Little Thing, she stumbles upon an ancient magic with the power to change everything! What to do world peace? Win back her oh-so-perfect ex? Well, if underthinking got her into this mess, her habitual overthinking should be able to get her out of it! When the fairy king Oberon is released from his enchanted imprisonment, he offers Kelly the simplest of gifts- Three Little Wishes. That's all, no need to think too hard about it. Just wish for what you've always wanted and he'll give you exactly that. Exactly, to the T. . . what could possibly go wrong?