Sharpe's Eagle is the first book in the series that Bernard Cornwell wrote (though not the first chronologically), and in the foreword he claims that he has not gone back and re-read it for fear of spotting crudities in his early writing.
However, the only thing that I would be worried about is the details about the characters which have been made incorrect as the series has developed, and that is unavoidable. What really matters is that the book is of the same high standard as found everywhere in the series.
Cornwell has the knack of providing plenty of background information, while not letting it flood the actual storyline. His attention to detail and description is beyond belief, and with almost casual ease he can place vivid images of the surrounding scenery and action in the reader's mind. His choice to have most of Sharpe's actual enemies, the ones he interacts with most, within the British army is inspired as it makes for a far more realistic experience and also provides Sharpe with a harder struggle to overcome. Sharpe has his foes amongst the French, make no mistake, but in Simmerson Cornwell is able to demonstrate the unfair system of promotion and power of the politicians in this period of British history.
I also liked the way that Cornwell doesn't paint the French as scruffy, ill-disciplined ruffians, in the way that a film-maker might be guilty of. This firstly makes gives the book another aspect of realism, but also makes Sharpe's victories more remarkable.
Finally, after the book, an appendix is provided by Cornwell in which he explains what actually happened, and how he altered events (subtly, I might add) to suit his story. This, as well as providing a useful history on the actual events, also goes to show just how well the author has researched his work.
I cannot recommend this enough.