Some simple rules to follow for improved typography (web or print), from Fred Design:

• Don’t use too many typefaces (not more than 3).
• Pay close attention to the hierarchy.
• No more than 4 font sizes, preferably 3.
• 8-10pt for body copy (definitely not above 12pt).
• Use simple, legible typefaces.
• Keep leading in mind (a little above the font size, slightly below the auto setting, no overlapping).
• Keep kerning in mind (no overlapping).
• Emphasise subtly (bold, italic, underlined or a darker shade).
• No caps in body text.
• Always align to a baseline.
• Flush left, ragged right (keep text left aligned, not justified).
• Between 6 and 12 words per line (as a general, flexible rule).
• Hanging punctuation marks and bullets, not indented.
• Experimental: use Fibonacci numbers for point sizes.

Similar and worthwhile: Mark Boulton’s Five simple steps to better typography.

Don’t forget these ebooks, either!