I have a graduate degree in Immunology, and I swore by Janeway for my coursework. The beauty of this book is that, although it does go into excessive detail (for medical school purposes), there are headings above each paragraph/topic that tells you what it is about. You can then either delve into more detail by reading the text, or you can skip to the next heading. Also, the illustrations are fantastic - you could get the gist just by reading the figures and then simply skimming the text.
Peter Parham's "The Immune System" is basically an abridged version of Janeway. It has plucked out the important concepts and figures, and thus does not delve into the level of detail that Janeway contains. It's a great introductory text, and has all of the benefits of Janeway's great figures without all of the extra wording and detail.
I still have both texts, and plan on using Janeway this fall in my medical immunology course.