Rebecca is a fantastic 'mood' novel: it's suspenseful and ensnaring without being obvious and brash. du Maurier adeptly chooses diction that conveys the tone and scene and draws the reader in, seductively but mysteriously, much like how the second Mrs. de Winter must have felt upon arriving at Manderley. It's a very easy book to get lost in. It's not a proper mystery, in the sense that there isn't a case laid out that requires solving, but our young protagonist does by necessity unravel the truth behind Rebecca's legacy and her marriage to her mercurial husband.
The second Mrs. de Winter is, literally, nameless, since her whole being becomes entangled with her husband and Manderley. She's a hopeless romantic -- young, naive, and eager to please. I don't recall her age ever being explicitly stated, but she is probably in her late teens. As romantic as this novel is in its idyllic descriptions of Manderley's gardens and great rooms, Maxim de Winter himself leaves a lot to be desired as a husband. du Maurier skillfully allows for the reader to recognize Mrs. de Winter's infatuation with the man as a hallmark of her youth and inexperience, even through her breathless adoration, for he really is quite sullen and condescending, and not the type that many young women would rush to marry were it not for the promise of the marriage plucking them out of some kind of dreadful current situation (as was the case with MdW2.)
Of course, such a marriage, even without the specter of a seemingly perfect dead first wife, will tend to change a person. Though MdW2 spends much of the novel feeling hopelessly gauche and undeserving of her husband's love, the secrets that are revealed to her cause her to grow and gain wisdom almost instantly. As such, I also relate to her loss of innocence directly correlating with her increase in confidence and, yes, maturity.
This is something of a classic and a novel that I'd definitely recommend to anyone, particularly those interested in historical fiction.