This book is about Citrix Presentation Server 4.5. (Called "CPS 4.5" or simply "PS 4.5" throughout the book.) CPS 4.5 only runs on Windows Server 2003, so therefore this book only discusses Server 2003.
CPS 4.5 can trace its roots back to Citrix MetaFrame XP, released in 2001. Most of the concepts in this book are applicable to any version of MetaFrame / Presentation Server / XenApp, but it's specifically written for Presentation Server 4.5.
There is another, newer version of Presentation Server, called "XenApp." XenApp is nearly 100% identical to CPS 4.5. The biggest difference is that XenApp runs on Windows Server 2008, while CPS 4.5 only runs on Windows Server 2003. In fact there is no version of XenApp for Windows Server 2003. If you want XenApp, you have to go to Server 2008.
Citrix keeps PS 4.5 up-to-date with "Feature Packs." The Feature Packs add little features and small upgrades to the products. At the time of writing, Feature Pack 1 was the newest Feature Pack for PS 4.5, so I mention it where relevant. Citrix is planning on releasing additional Feature Packs for PS 4.5 over the years to keep the features more-or-less inline with XenApp.
Also, Citrix Presentation Server 4.5 comes in several Editions: Advanced, Enterprise, and Platinum. This book is generally written for the "Enterprise" version of Presentation Server 4.5. That doesn't mean that it isn't relevant if you have Platinum edition though! The problem with Platinum edition is that it's not really an edition of Presentation Server per se, it's actually a "suite" that includes several other, non-Presentation Server products (like Citrix EdgeSight, Password Manager, etc.). I can't possibly write a single book about the entire suite of software, as that book would be about 3,000 pages. So this book is about Presentation Server only, but it covers the "Presentation Server" that's part of Platinum or that stands alone as "Enterprise" or "Advanced."