As interesting as it is to read a biography of a queen as important to English history as Isabella, Weir's treatment should be viewed with caution. As she always seems to do, Weir slants the history and stretches her interpretations to the breaking point in order to "prove" her theories. For example, she veers sharply between stating that Isabella was a strong and confident woman and then, a few pages later, insisting that she was dominated entirely by Mortimer, then very soon she's back to being in control.
Of particular interest in this case is her theory that Edward II was not murdered but escaped and survived to die of natural causes in exile without his identity ever being acknowledged. While it is possible this is the case (though I believe it to be unlikely), Weir provides the evidence, admits that it is not conclusive, then proceeds to act as if the matter had been settled favorably and future events are interpreted as if his survival was a given.
So by all means read and enjoy the book, but maintain a healthy skepticism over her claims in favor of Isabella being a good woman whose only flaw was greed.