I have very high expectations for Giardina. Maybe I shouldn't, but if she could write such stellar books as Good King Harry and Storming Heaven, she doesn't have much of an excuse for this.
Truth be told, Fallam's Secret isn't all bad. It has a really strong beginning setting up supernatural intrigue and developing an interesting fantastical premise based on obscure theology. The setting in seventeenth-century England is fairly interesting as well, and the contrast between the modern woman and puritan society makes for good drama.
The problem with this story comes from the contrived romance. Without spoiling anything, I'll just say that the protagonist gets with a mysterious man who definitely has attractive and interesting qualities, even though his affections for the heroine aren't terribly believable. The two basically fall in love instantaneously, which is made especially strange given the fact that our protagonist is disguised as a boy. There was a lot of potential here to develop a unique and compelling romance, but that was thrown out in favor of what feels like trite romance novel garbage.
This book is far from the worst thing even, but for Giardina it's pretty disappointing.