Here’s a list of pages I find interesting (occasionally steal inspiration from):
A belgian chemist and optical engineer who does many different things from counting bubbles to building thin film refractometers.
A subsection of the astrosurf site with a lot of information for building spectrometers. (There’s lots more on the astrosurf site itself.)
T.J. Nelsons site contains information about lens and spectrograph design.
The list is so I don’t forget about these resources (because I’m sure I already forgot some). Feel free to send me more links.