Relativity by Einstein might be a good choice.
Thank you. Is there a book that maybe explains what the study of physics is and where the field stand today?
The “Feynman Lectures on Physics” is the all-time classic. Caltec owns rights to his books, probably available through Amazon, but much can be found on-line through a search on the quotes. Einstein’s books require too much knowledge in mathematics like the Taylor Series Transform for most to begin to understand. The Feynmann Lectures were more of a classical physics for the non-physics PHD.
It's more his style than the difficulty. Also relativity theory is one of the last topics an aspiring physicist should study.
You haven't indicated your wife's level of education, nor did you say much about her personality. Is she obsessive or is this a passing whim?
If she knows high school physics and passed a course in calculus, a good college textbook (Resnick and Haliday is excellent) is the *perfect* place to start.
If she's a little less sophisticated, pick up a high school textbook.
She might benefit from reading periodicals, like The Physics Teacher which is aimed at high school physics teachers. The articles are stimulating, relatively (that word again!) straightforward, and a beginner can usually follow the gist. The important thing is to maintain one's interest.