Long ago, before access to the Internet was cheap and plentiful, and way before search engines made finding this kind of information easy, this was a priceless find for an aspiring low-level programmer. All the (semi-)common PC hardware and software documented in one place.
Endless hours spent exploring VGA hardware registers and trying to apply them for cool visual effects. Learning how the then-new 32-bit Windows interacted with DOS extenders, and trying to make a homemade - very basic - operating system that could do it, too. The thrill of writing a Terminate and Stay Resident alarm clock, and having it finally not explode...
I have very fond memories of the Ralf Brown's Interrupt List.