Ash is one of the greatest fantasy novels ever written. There may, it is true, be some argument about its status as fantasy, rather than alternate history or SF - but, whatever its genre, it is one of the greatest novels of that genre.
Ash, the heroine, is a late 15th century mercenary captain, and the novel takes place in the final months of the Duchy of Burgundy. At first, with a couple of very minor details (not unparalleled in actual chronicles of the era), the novel seems to be purely historical - but then, gradually, things change.
The novel keeps moving through over 1100 pages without faltering: there is scarcely a line, let alone a page, that does not advance the story - in ways that always arise from the story but can still often surprise the reader. The characterisation is superb, with a realistic account of the difficulties that a mercenary captain of the time (especially a young female one) would have had. Historical facts, when not intentionally (and clearly) changed, seem to be uniformly accurate: Mary Gentle's research has obviously been thorough.
The one partial exception to the above, in my view, lies in the fact that the novel is presented as a near-future edition of a series of accounts of Ash's life, with both editorial footnotes and a series of letters and emails relating to the edition's publication appearing between the chapters of the novel. Here, the characterisation is rather weaker, and the reader may be tempted to skip them. However, this would be a mistake: they are essential to the final resolution of the novel.
To conclude: if you buy just one fantasy novel this year, buy this one.