What is considered optimal word spacing, letter spacing, characters per inch etc. for best readability?
Are there studies with results that anybody can cite?
According to Robert Bringhurst, author of The Elements of typographic style, considered by many as the typographer's bible:
Anything from 45 to 75 characters is widely-regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal.
There is a similar question with some comprehensive answers.
See this WCAG guideline on line spacing http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-visual-presentation.html
I would add also that reading is a learned skill, to the point of being automatic for literate people. The brain handles reading differently when it has become automatic. But we are very adaptable. We can learn to read all caps, various fonts, etc. just as easily. We can even read when all vowels are removed.
What this all means is there is no optimal size or spacing as far readability or perception is concerned. What is optimal is to give people what they are used to. So if your text style falls within the general norms, then the automatic process will engage. If you try something radical, you will force people to think.
I believe most guidelines you find will try to codify this idea of what is typical.
I'm not speaking of comprehnsion, which is a separate issue (meaning vs. shapes)