Like most of her books, this one is said to be an interesting, well written read but not so reliable as a a guide to what actually happened. Professional historians who are specialists have never had much use for her because she writes for a popular audience and is not as familiar with the materials as she should be, thus being free to imagine some things which those better versed know to be unlikely or suspect.
In this case, professionals reviewing the book say she overlooks some of the very serious dislocations that happened as a result of Lincoln's appointment of his adversaries to key positions. It was not nearly as effective as she paints it.
Their main problem is that she has been caught plagiarizing other people's work. This is well known in the historical community but rarely mentioned when her best selling books come out and get reviewed by non specialists.