Tutorials and documentation concerning maths with LaTeX:

• Mathmode
  by Herbert Voss
  Extensive collection of materials concerning mathematics with TeX and LaTeX, highly recommended
• amsmath user’s guide
  American Mathematical Society
  Documentation for the math package
• Math into LaTeX
  by George Grätzer
• Short Math Guide for LaTeX
  by Michael Downes
  concise summary of recommended features in LaTeX and extension packages for writing math formulas
• Using the amsthm Package
  American Mathematical Society
  Enhancements of the LateX theorem environment
• Höhere Mathematik
  chapter 8 of the german edition of the LaTeX Companion by Mittelbach, Gossens
• Mathematik mit LaTeX
  easy introduction by Jana Peters