a) Overview of mature vs. immature detrital rocks: compositionally mature & texturally mature (big file so loads slowly)
http://www.usouthal.edu/geology/haywick/GY402/402-pp11.pdf

b) Overview of med-grained detrital rocks in thin section
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_1.htm

c) Thin section "catalog" of various mineral grains in detrital sedimentary rocks
(big file so loads slowly, most text in the first few frames is too small to read and too low resolution to magnify, but the pictures are FANTASTIC)
http://www.geology.ukzn.ac.za/staffweb/hoffmana/thin_sections.pdf

d) Thin sections of 15 common lithic minerals; useful for identifying grains in detrital rocks
http://geology.fullerton.edu/bbrowne/303B%20Rock%20Forming%20Mineral%20Atlas.pdf
(Note: These are from relatively fresh (unweathered) igneous and metamorphic rocks. Detrital grains will have been weathered, making identification more challenging)