The title is clear and well defined and precisely describes the subject matter - this book is entirely focused on developing Visual Basic (VB) Integrated Development Environment (IDE 5.0&6.0) add-ins using its extensibility model, it is not aimed at developing, via add-ins, an extensible application.
For those using Visual Basic and wishing to extend the IDE there is information scattered around, but not in one handy reference - until now. The book is clear, well laid out and with plenty of short code snippets that illustrate the text. The book develops its theme from what is an add-in through to using VB IDE's object model such as its menus and toolbars. The sequence is logical and straightforward such that you are easily led forward into more and more complex areas that build on the previous sections. The language is simple and there is good use of diagrams - in short for developing VB add-ins it is a useful book to have and to refer to.
For those interested in developing applications with an open model that can be extended, such as Adobe Acrobat or the Netscape and Explorer browsers extensibility model, via plug-ins then it would probably be best to look elsewhere. The book is very well defined and targeted and on that basis although there may be some value in trying to translate the concepts it would be simpler to look elsewhere.