Enchanted, Inc. by Shanna Swendson
Katie's been living a year in NYC, getting used to the weird, which is why she can wearily comment on the woman wearing fairy wings and who had to be wearing shoes with colorless high heels as she treks to her horrible job with a horrible boss. But more people pay her attention, and she starts getting emails with odd job offers.
So she investigates and gets a job with Magic, Spells, and Illusions, Inc. Because, it turns out, she's immune. She's not affected by magic or illusions. And, it turns out, not only is the magic business in need of more modern methods, they have a problem ex-employee. Marketing when you can change the packaging that's already on the shelves, turning people into frogs, Merlin and why you can't use him as a celebrity endorser in your marketing campaign, a boss who sometimes turn into an ogre, and more all feature.
Once Upon Stilettos by Shanna Swendson
The second book.
It suffers a little because the plot turns on Katie making a stupid decision -- her immunity goes away, and she doesn't tell -- and while she gives a reason (fear of losing her job), it didn't convince me strongly enough.
Because the lawyer's letter last book only prevented the problem ex-employee from using what he learned on the job with them, more problems arise, and Katie is in the thick of them. It involves red shoes, discovering that her mother's immune, someone selling pumpkins, a need for morale boosting, helping a shy male co-worker buy for his foster parents, and more.