Science and Mathematics Initiative for Learning Enhancement (SMILE)
Smile: Polyhedrons
This in-depth paper examines the idea of convexity. Euclid's Elements, the first geometry textbook, assumed convexity without mentioning the term.
Other
Mathematics Internet Library: Free Internet Books
Practice problem worksheets are available for students needing to study basic terms relating to triangles and other math concepts.
National Council of Teachers of Mathematics
The Math Forum: Pictures of Icosahedrons
This site from Dr. Math and the Math Forum provides several links to pictures of icosahedrons. The information is somewhat brief, but worth checking out for a background on the subject.