I have used the original ksh[1] for years, because it seem to have more features for complex shell scripts. It also supports real numbers for people that need math functions other than integer.
The ksh that ships with OpenBSD, it's available under Linux as loksh. I've never needed a tutorial as such, because found its man page to be well written and comprehensive. However, I can recommend the old book The Unix Programming Environment by Kernighan and Pike, it also is well written and helped me quite a bit. Just understanding one smaller tool (ksh) better has helped me do more with it and use fewer addons extensions, which personally make me happy; works for me.
[1] https://en.wikipedia.org/wiki/KornShell