You'll find a lot of information all over the internet, but I'm not sure where the information is presented in the most concise manner.
There were several books on the 6502 and some that were quite good, they will still be in some libraries but I expect most would be out of print.
Here's a place that I judge to be a decent introduction to the 6502 but it's not a course so it goes pretty fast in sections.
http://www.obelisk.demon.co.uk/6502/
A site that has a lot of good 6502 code but a lot of it is C64 oriented and you have to wade through a lot of discussion and unimportant information.
http://www.ffd2.com/fridge/
I haven't tried this yet but if it's decent it will be a good way to experiment.
http://sourceforge.net/projects/sim6502net/
Also, you could also post questions here on this forum!
Good luck!