Be Careful
At least one major historian regards this life of one of the major figures in pre-Revolutionary America as mainly fiction because of the author's heavy reliance on Irish folkways. But at times it can be interesting and suggestive.
Sir William Johnson was a complex man who lived successfully among the Iroquois, the British, and the colonists. Previous biographies like Milton Hamilton's are good but a modern one is lacking. This isn't it unless you too are heavily influenced by Irish legends.
It's too intense for a summer read or a beach book and it fails to satisfy the historian; sources are dated especially secondary ones for background.
I'd wait for the forthcoming major study on Johnson.