I have collected several useful manuals on TeX and LaTeX:

• Plain TeX:
  • A Gentle Introduction to TeX by Michael Doob, 91 pages.
    Download 250K DVI file
• LaTeX:
  • An Introduction to TeX and Friends by Gavin Maltby, 75 pages.
    Download a 212K DVI file
  • Document Preparation With LaTeX edited by D.Budgen and S.Nelson, 46 pages.
    Download a 640K PostScript file
• LaTeX 2e:
  A not very short Introduction to LaTeX 2e by T.Oetiker, H.Partl, I.Hyna, E.Schlegl, 58 pages.
  Download a 518K PostScript file
• AMSTeX and AMSLaTeX:
  • Users Guide to AMSFonts
    Download a 140K DVI file.
  • AMSLaTeX Version 1.1 Users Guide
    Download a 210K DVI file.