One Day in December is written in a rambling, conversational style which often reads as gossipy, but then at times it seems as though this is the only way that it could have been written, relying heavily as it does on, well, gossip (or more politely put the anecdotal stories of family, friends, and comrades). This I don't criticize, exactly, however it does lead to the inclusion in the body of the text information which would have been more befitting of a footnote (there are none), as well as asides which often appear tenuously relevant if at all.
The portrait presented of Celia Sanchez herself is well in line with her popular image and legacy, which I enjoy, however at times it did seem as though the author was shoehorning events into an image. This usually takes the form of surrounding core facts with speculation as to what Celia must have thought or felt, without any supporting information beyond it being "the sort of person she was," where that "sort of person" upholds a number of markers of being mythologized. (The Celia described was always strong, always kind, always generous, always just, etc.) Still, I am an admirer, which makes it difficult to come down too hard on a portrait which may sometimes glow a little too brightly.
Where I did find the content frustrating was in its often disproportionately heavy focus on Celia's personal style, rumors surrounding her relationship to Fidel, and the "feminine touches" she brought to clandestine service and war. I understand that this framing of her life is embedded in her legacy, but there were points at which it seemed over the top or incongruous with events to the point of having left me (possibly bitterly) saying to myself that a biography of a man would never have been written like this.
Overall, however, the book is filled with stories and anecdotes not easily found elsewhere in English, as well as generous photographs spanning her life (though some are described but not shown, leaving the reader wanting), and among the not-always-relevant asides there are a number of interesting stories I'm not sorry to have read. It's a good book; it simply could have been a better one. How much better is, in part, a matter of tastes.