When you have a history book with an opening chapter that talks of Stone Age man arriving in England, you know you're onto a loser.
This is one of many errors, omissions and fantasy that pepper this so-called 'essential guide' to Cornwall's history.
So we have barely anything on the Cornish language; Anglo-Cornish wars reduced to mere rebellions, with key Cornish players hardly mentioned; constant erroneous references to Cornwall as a County etc. Nefarious characters like the appalling Anthony Kingston, who along with his death squads, was responsible for the slaughter of hundreds and hundreds of Cornwall's population warrants only a few sentences. People like that (and their masters) changed Cornwall's history forever.
History is there to be had and there's no excuse for misinformation and shoddy research/writing which I'm afraid this book has in spades.
For a more factual, indepth (though not perfect) look at Cornwall's history and constitutional status, Philip Payton's Cornwall: A History
is to be recommended over Halliday's.