As with many books of this nature, Understanding COM+ is truly geared to a breed of developer/analyst interested in learning about the strategic, rather than the physical implementation of Microsofts COM+ technology. A Prior knowledge of Microsoft Tranaction Server, and COM under Windows NT v4.0 would be advisable, however, the information is provided here - if you have the patients & resolve to re-read certain early passages 2 or 3 times.
The subject matter is by nature rather an obscure one which the book acknowledges, but strives to draw comparisons between itself and and its ancestory with a good degree of success; dealing with the underlying concepts in a comprehensive and tangiable fashion.
Microsoft Press are producing books like this more readily nowdays, as their underpinning technologies become more abstract from the user / developer. Keeping this in mind is important, because those hoping to find reams of C++, Visual J++ or Visual Basic code / examples will be disappointed (these are found on the books website references).
To fully exploit the potential this subject matter provides, I recomend viewing "Designing for Scalability with Microsoft Windows DNA" isbn: 0-7356-0968-3, as this book also complements a knowledge of COM+ and deals with one of COM+'s strongest fields - Distributed Application Development.
I have found this book most useful in the context of of a developer migrating from NT 4.0 to NT v5.0 (Windows 2000).