The Masqueraders by Georgette Heyer

Prudence showed an impassive face to John who was waiting to let her into the house, but she slipped past the door of Robin's chamber on tiptoe, and was gone into her own without the usual visit to him. She preferred to meet her sharp-eyed brother in the morning, when she might have acquired some command over herself.

Sure the world was upside down. And who would have thought it of the large gentleman? She had come to think she could no longer be surprised, but this strange proposal of his came to dispel such fancies. He meant it, too: not a doubt of that. As she prepared for bed she thought over it long, and with some agitation. The gentleman's last words lingered; they had been forcefully uttered; she believed he was not the man to promise what he would not perform.