If you want to understand what's going on inside an Intel-standard PC, Mueller's tome should be at the top of your hardware-book heap. Upgrading and Repairing PCs overflows with both tutoring and reference material. Mueller intersperses intelligent and easy-to-follow prose with conceptual diagrams and drawings and factual tables. You can either read through and learn from the author's experience or flip right to the answer you need to solve a problem. The up-to-date information includes Pentium II and limited coverage of Celeron, Xeon, and the latest non-Intel processors. The author also accurately surveys state-of-the-art components aside from CPUs, covering new SCSI boards and Voodoo Rush video, for example. The guide hosts a directory of almost every company that makes hardware for the PC, complete with physical addresses, phone numbers, and URLs. On top of that, there's a good glossary, a table of modem commands, and thousands of hard-drive parameters (heads, cylinders, and so forth). Two companion CD-ROMs hold a searchable database of hard-drive specs, some videos of Mueller's lectures, and a bunch of diagnostic software. Though it's not specifically written as an A+ certification study guide, this book could be used as preparation for the hardware portion of the A+ exam. Mueller's coverage is far more comprehensive than what that exam requires, so be careful about studying details that don't appear on the test. --David Wall