I collect old computer manuals and related books, mainly because I started buying them when they were new and have never really stopped.
Publications like the Inside Macintosh series, my copy of the rare "Commodore Plus-4 Programmers Reference Guide" and various versions of "Apple Macintosh System Software Users Guide" were acquired as they were published or came with systems I have bought over the years.
Others like my Apple 2 System/DOS/Programmer's Guides, original IBM PC manual kits and seminal books like "Algorithms + Data Structures = Programs" as discovery and money have permitted.
Best places I've found for these things are charity and second-hand stores. They get some extra money for their projects, I get "antiques" in print-form - it's win-win!