The Paper Princess by Marion Chesney
A fun, light read laced with some gentle satire – society believes Felicity’s masquerade because no-one wants to risk looking stupid by admitting they’ve never heard of ‘Brasnia’ or its royal family. Several characters – the stepfather, the butler, the stuck-up fiancée – deliberately lean towards caricature, and Miss Chubb(inski) is clearly living her best am-dram life. The hero is aware of Felicity’s secrets pretty much the whole time, so there’s barely any conflict caused by that.
All in all, it’s cheerful braincandy, and well worth a read if you’re looking for a light-hearted Regency romance.