Guide to Thin Section Microscopy (2nd ed., 2012) by M.M. Raith, P. Raase, J. Reinhardt
https://www.minsocam.org/msa/openaccess_publications/
or from
https://www.dmg-home.org/service/publikationen/downloads/
Handbook of Mineralogy by J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols (eds.)
https://handbookofmineralogy.org
Olaf Medenbach's website on mineral optics
(includes excellent presentations on conoscopic interference figures, including dispersion and various anomalies)
https://homepage.rub.de/olaf.medenbach/eng.html
Frank Mazdab's rockPTX mineralogy-petrology website
https://www.rockptx.com
Virtual Microscope site, Open University, U.K.
https://www.virtualmicroscope.org
Virtual Petrography (ViP) - A virtual microscope for the geosciences, RWTH Aachen
https://publications.rwth-aachen.de/record/675017