I'm reading this book at the moment and I agree that it's poorly written and not really up to scratch. But regarding your comment that Warwick the Kingmaker was the brother of Cecily Neville - actually, he was her nephew, the son of her brother Richard Neville, Earl of Salisbury (Warwick was also named Richard), and thus a first cousin of Edward IV and Richard III.
I'm also finding it irritating that she quotes huge chunks of text from chroniclers and other historians in the middle of sentences - as if she couldn't be bothered relating what happened in her own words. Why bother writing a work of history if you're going to quote and quote and quote from others? And she rarely names the source, so that I have to keep checking the end notes to see whose books she's quoting from.
The Real Queen of France: Athenais and Louis XIV was brilliant. Hilton must be far more suited to writing proper historical biographies, rather than history books that contain simple and uninteresting sketches of a lot of historical figures.