And the early BBS's, dialing up on a 1200 or 2400 baud modem to your local boards was pretty common, and every one of them had a file sharing service with a ton of fairly common files. Things like Ralf Brown's Interrupt List were standard to find everywhere, as was the code from a bunch of the more popular magazine articles. Many snippits that later became the PCGPE or lead to what was in the PCGPE, those were floating around the various sites and boards, and even the big services like Prodigy and AOL in the late 1980s had collections to browse.

Again, it wasn't books. It was technical papers, code snippets, pieces of documentation that were 2-10 pages long. They were everywhere, the hard part was sorting through them to find what was immediately useful, and saving what would be useful at a later date.

I still have a few in my desk drawer from that era, that were too sentimental to toss into recycling. I would print a bunch of them to read, printed multisheet 2x2 per page on the old dot matrix printer, which I would mark up and annotate. Converting from segment:offset layouts, CPU instruction timings for the 286 and 386, the transition from 16-bit to 32-bit processing and compiler's default data types, collections of sorting routines (several pulled from Robert Sedgewick's first edition of his now-ubiquitous algorithms book), memory addresses for various hardware systems, and so on.