One of Danielle Steel's popular romantic novels is 'The Duchess'. It has elements of undercover in the sense of hidden emotions and pasts that gradually unfold within the romantic storyline.
Another great work is 'Zoya'. It's a story full of passion and mystery. The main character has a past that is gradually revealed as the romantic plot develops. It gives a sense of an undercover aspect as not everything is known at the start, and as the reader delves deeper, the true nature of the relationships and the characters' stories come to light.
Danielle Steel often weaves complex relationships in her books. For example, in 'Message from Nam', there are love stories that are somewhat like undercover romances. The characters have their own secrets and internal battles while falling in love. The way she portrays the era and the emotional turmoil of the characters makes it a great read.