The book is a fair and balanced description of his life. It covers everything from his young life to his development of Mellon Bank, the United Trust Company, Gulf Petroleum and Alcoa and his experiences as Treasury Secretary.
The book does less well at explain how he managed to be Treasury Secretary for so long (almost 12yrs). He regarded the country as a business. If you lost your job, his attitude was to "suck it up and pull yourself up by your boot straps," an attitude which was unlikely to endear him to politicians.
In addition, while his shyness, reserve and inflexibility of expectations might have worked when he was his own boss, I struggle to se how they would work when working with a separate but equal arm of government, who need to be stroked fair deal to ge things done. Mellon struggled and yet was kept in place for a good while, which is something that I don't understand (and isn't very well explained in the book -I doubt it even dawned on the author).
Given that this was the first biography of Mellon, I'd say that it was a pretty good book, but don't expect it to answer all the "whys?" you might have about Mellon