The main thing I garnered from this book is how many areas are directly or indirectly effected by ADSI. ADSI is not just for Microsoft Active Directory; ADSI supports LDAP and Novell NDS directories as well.
ADSI can be accessed using Microsoft COM+, OLE DB, or even ODBC. This book touches on each of these access methods, but not in great depth. For instance, the section on ADSI and Exchange Server, IIS, and Site Server, is a scant 30 pages.
"Professional ADSI Programming" is ideally suited as a roadmap to guide you to specific areas of ADSI development. You will need specific references, such as Microsoft Developer Network (MSDN), to provide the detail absent from this book.
At times the author gets side tracked from the topic he is trying to cover. This is more the fault of the editor than the author.