Incidentally, this reminded me of Starkey's biography of Elizabeth. Nothing against (most) of the book (although it's been an age since I read it), it's an easily readable account of her formative years. There is nothing wrong, either, in limiting the scope of your book. What rankled was in the introduction where he set out his rationale for concentrating on the years before her accession. Historians, he says, often fall in love with Elizabeth but he dismisses the "bewigged and beruffled Gloriana... with her face caked in carmine and white lead... This Elizabeth interests me but rather repels me"
So, the apogee of one of the most successful women of her age or any other dismissed because Starkey thinks she got old and ugly and therefore "repels" (worth repeating!) him. Had he just written that he was curious as to the influences and experiences that informed the monarch, then fair enough. But he had to make it about how personally unattractive she became.
The rest of the introduction annoyed me too, featuring as it does plenty of typical boasts about how original he's been and how much cleverer he is than his readers (including the ludicrous suggestion that readers will see the very famous portrait of the 13 year old for the first time on his book cover).
Oh dear, now I better calm down for a bit!