This dictionary is absolutely one of the most useful dictionaries of Celtic mythology around.
It has great summaries of all the important and also the less wel-known stories.
For the rest it contains entries for every charcter in celtic mythology, for most important animals and plants, for place-names, ... But also for continental gods, figures of popular Cornish, Scottish or Breton culture, important events in Celtic history, and so on.
Every important entry ends with a small, but very useful biography, he even lists which modern writers were inspired by the particular story or character.
The book also has a small, bit conservative foreword, that I have found less usefull than the main part. It ends with a great thematic index of entries, e.g. animals, irish texts, continental gods, ...
In short: this is a real must have for everyone with a serious interest in Celtic mytholgy or culture.