C64 manual in German (I only knew Polish then). I typed some of the programs into the computer, and played with changing the values.<p>I remember to this day that graphic programs using sprites didn't worked for me (must have been some print error in manual, I've tried like 100 times to run that program - it was supposed to draw baloon, but it just did nothing).<p>Then I got huge red book "Mikrokomputery" by Herwig Feichtinger from my father. It had schematics and opcodes for Z80 and 6502, listings of commands with short explanation for a dozen variants of basic, pascal, forth and c, commands for CP\M and DOS, and some nice algorithms (drawing circle pixel by pixel, questions and answers program in basic using "learning" decision tree, some utilities to format a disk, etc). And it was in Polish (translated from German I think), so this was the first programming book I could understand.