It's not very often that one finds a science textbook, let alone and immunology one, that it is possible to sit down and read, chapter by chapter as one would with fiction. Kuby's Immunology is an obvious exception. The book is divided very logically into chapters which cover (more or less) one topic at a time; allowing you to read about one subject in one sitting, without having to check back in 7 other parts of the book or going off on a tangent.
The diagrams contain no more information than they need to and support the text very well. You can revise from the book quite easily using the diagrams alone as a guideline. The colourscheme of the book keeps things simple, but does make things look a bit dull at times but then that's immunology for you.
The downside? Not enough journal references!