How might an instruction manual designed for troubleshooting differ from an instruction booklet designed for improving performance in a skill?

Answer 1

I don't know that they should be different.

The fonts chosen should be clear and easy to read. I'd use a sans serif font for chapter and section headings and a serif font for the text of paragraphs and instructions.

Any illustrations also need to be crystal clear and easy to understand.

Answer 2

http://en.wikipedia.org/wiki/Sans-serif
and
http://en.wikipedia.org/wiki/Serif_font