I used this and a DK guide on Umbria and Tuscany as appropriate. They complement one another well. The Cadogan Guide is full, clear and well written and much more satisfying than most other guides. However the maps are, I have to say, dreadful and should not be relied upon for anything more than casual and limited ambling. The map on Siena is a point in case. Even allowing for the paucity of the practical guidance on how to get from one of the car parks into the city centre, the map provides no reassurance and the complete absence of any informative (as opposed to mood-enhancing) photographs is frustrating.
In short, this is a valuable resource but I strongly recommend acquiring a much more visually arresting guide with better maps and cross-referring between the two