[Jellby]

Not being a native English speaker, writer or reader, these are my answers:

Originally Posted by radius

- on US keyboard, is the key next to the semi-colon an apostrophe, a prime mark or single quote? Which glyph should be used for contractions and possessives?

I don't have a US keyboard, but I guess it's a straight single quote (the one I'm using here). In a text with any kind of typographic self-respect, use ’

- Rules about quotes inside quotes; assuming conversion to left/right single quotes, do you space between the double quote and single quote? If so, is this a  ?

No space. This should be a job for the font's kerning.

- when using a dash to separate clauses, which size do you use? Do you put spaces on either side?

Either — with no spaces or – with spaces. I tend to favour the latter in ebooks because it causes less line-breaking issues.

- how about space after ellipses? Or only space after if it would be the end of a sentence?

. . . with spaces around. Turn one of these spaces into   if it's something that shouldn't break (e.g. . . . ?)

- When italicizing, do you include the punctuation for a sentence. I assume this doesn't include the enclosing quotes for dialogue?

I italicize what is meant to be italicized, not what just happens to be next to it. No punctuation unless it's part of a full italic sentence or something like that.

- For quotes that aren't part of dialogue, does the punctuation go on the inside or outside of the quotes? For example, should the question mark be on the inside of this "quote?"

Same as above, punctuation outside.

- If you use indented paragraphs, when do you have a non-indented paragraph? I usually do for start of chapters or scene breaks, but how about after an illustration? Or a quote, letter, list etc. of some kind?

In English texts, at the beginning of some kind of section or break in the narrative, not just because there's something a non-text element above. Or if it's a "fake" paragraph break (the same paragraph continues from the preceding text).