There can't be any information about volcanology pre-2000 that is not included in this book - it really is an encyclopedia.
Every topic in physical volcanology, volcanic hazard, and all related subjects including volcanically formed ore deposits, volcanoes in books and films, ad infintum appears to be covered. Each chapter is written by an expert in the field which really adds to the quality, as well, as a result the book appears to include a section by every volcanologist in the world which again is very useful if you're involved in volcanological research.
The technical detail is excellent and where it stops there are a series of useful references (usually journal papers). Each chapter is about the right length to dip into when requiring some expert knowledge on a particular aspect of volcanology. The only slight downside is that because each chapter is written by a different author the style varies a lot, however all appear to be written to a high standard.
This book is a must for anyone seriously studying volcanology, for whom I'm sure it will be an indispensible reference guide. As I final recommendation I have seen it on the office bookshelf of more than a few volcanologists!
A final word of warning however this book is very expensive and I have heard some unsubstantiated rumours that a new edition is in production, the current edition is beginning to age, so it maybe worth holding on for the new edition, if indeed the rumours are true!