1. Britney Spears' Guide to Semiconductor Physics: http://britneyspears.ac/lasers.htm
2. Franco Maria Boschetto's personal website[1]: http://www.fmboschetto.it/
3. Ripasso di matematica aka RIPMAT: http://www.ripmat.it/
[1]: hn discussion at https://news.ycombinator.com/item?id=22326339