Your first thought might be, what has MS-DOS got to do with anything these days, but when you consider the huge existing install-base of Windows 95 and 98 machines, plus the number of problems you can still have with Windows that can be solved with a bit of command line typing, you'd be surprised at what a handy reference this book can be.
In its 20th year, Running MS-DOS is a one-stop guide to the operating system. Covering the basics from how the OS works to tweaking it for better performance (HIMEM and EMM386 anyone?), it also delves into optimising your hardware, creating batch files and running legacy applications.
It's a thorough book, with a simple to follow design, and although it deals primarily with MS-DOS version 6.22 (95/98 use 7.0), it also works as a great point of reference for today's command-line operating system adjuncts.