Another site I discovered while searching for TMA (three mirror anastigmat) examples is Amateur Telescope Optics. Don't be thrown off by the word "amateur"- this is a very professional site with a tremendous amount of information, mainly about telescope optics: basic principles, design forms, aberrations, effects of aberrations, eyepieces, and even the human eye.
This site is not about how to buy a telescope. It's about how telescopes work at the optical level, how they are designed, and what the differences are between different design types. It has a lot of algebra-level math and many excellent diagrams and explanations. There's even an appendix with design prescription data for examples of the many design forms.
If you want a survey of telescope history and types with lots of pictures and diagrams (and no math), see Jim Burge's PDF/PPT (5 MB PDF).