For the right reader, _Lancaster Against York_ is a four-star book. If you're looking for an overview of/introduction to the political and military history of the Wars of the Roses, this is an excellent choice.
But there are several important caveats.
First and foremost: There is a titanic cast of characters. It is not always easy to remember who is whom, especially since the same personage may be referred to by first name, family name, or suffix, all of which are usually shared by several *other* characters. Trevor Royle chose to be complete at the expense, I think, of clarity. For example, there's a plethora of information along these lines:
"Leaving his brother George Neville, Archbishop of York, in charge of affairs in London, he [the Earl of Warwick] took his army north ..."
But George Neville, Archbishop of York, is not really relevant to the subsequent events. He and London alike could have been omitted from the passage quoted above with no loss of essential information. (There are already a raft of Georges, a horde of Nevilles, and a flock of Archbishops cluttering up the scenery; one more doesn't help.)
This is but one example. There are some authors who have mastered the technique of keeping track of this kind of multi-decade, multi-family chronicle--Robert K. Massie's _Dreadnought_ is a favorite of mine. Royle does not demonstrate that kind of authorial discipline. (There is, however, a helpful appendix of main characters in the back.)
Second: The subtitle of the book refers to "the Foundation of Modern Britain". This is arguably misleading. The book does not talk very much about the subsequent implications of the wars, or the social changes they introduced or reflected, or how they're relevant to later history. About all it does is give a brief summary of the later descent of the British monarchy. Otherwise, this is a strict blow-by-blow account of who did what to whom. (It is not, for example, a counterpart to Barbara Tuchman's _A Distant Mirror_.) Do not approach this book in hopes of learning much about the origins of the modern British state, or late-medieval arms and armor, or religious upheaval, or social conditions, or economic developments, or anything of the sort. Such things are treated, if at all, as minor adjuncts to the main chronology.
Third: I cannot forbear to mention some remarkably ham-fisted copy-editing. Some examples:
* p. 8: "The spark was the decision to raise the poll tax to one shilling (eight cents or $42 today) ..." What? Eight cents is not $42, today or any other day. If this is intended to equate one shilling to eight cents, it's gibberish: one shilling was *twelve* *pennies*, not eight (or any other number of) "cents".
* p. 78: Henry V's army en route to Agincourt "consisted of 105,000 soldiers". This is highly improbable for a medieval host. On the very next page, furthermore, we are told that Henry had lost a third of his fighting men", leaving him with ... 6,000. A few pages later, we are told that the French had 20,000 men, that they outnumbered the English by six to one, and that (once again) the English numbered 6,000.
* p. 320: Anthony Gray was the "4th Duck of Exete". Quack, quack.
Not all of this is necessarily Trevor Royle's fault, but it's all part of the book.
However: these (aside from the editing) can be seen as legitimate authorial choices. They're not necessarily problems, just features that will appeal to some readers more than others.
I'd describe _Lancaster Against York_ as, in effect, a very good "newspaper account" of the Wars of the Roses. Royle is excellent, and very complete, at describing the essential newspaper features of every battle and every political twist--who, what, where, when, and why. This is, perhaps, a good justification for including so many minor characters: they were there, they were part of the story, and some reader might want to follow up. Indeed, this would be a great book for someone who wanted to get a grounding for some further deep research.
Also, in spite of the cast of thousands, Royle does succeed in keeping his main focus on the various Kings of England and their immediate cohorts. This is conventional, perhaps, but it is essential in book about factional fighting for control of the English Throne. And it can be surprisingly difficult to maintain such a focus, particularly in a tale with as many turns as this one. Even if you find yourself losing track of the secondary players, the relationships and actions of the big names are always clear, and will let you follow the main thread of the story with no difficulty.
So I recommend the book--for the right sort of reader. It is what it is; don't go into it expecting something else, and you'll be pleased.