I personally would take a copy of Windows XP Annoyances with me when visiting a client on site because its more comprehensive coverage is more likely to provide the clues I need when trying to fix a problem. However, if I had a laptop using Windows XP (mine uses Windows 98) I think that I might well have a copy of this book in its carry case. There is enough in it to be helpful and even at 180 pages (note that, definitely still an anorak pocket book even though 50 pages longer than the PHP volume.) it is light enough to add to the essential gear for a laptop.
Windows XP Pocket Reference
Book cover image courtesy of Open Library.