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!