The default numbers in most typefaces are called lining figures and are designed to be used in columns so that we can add them up. The problem is that lining numbers stand out too much in a line of type.
In terms of bang for your buck, there's nothing like using old-style numerals for giving your type a more sophisticated look. Old style figures are as tall as the x-height of your type and so have a better type color in body text than full-height "lining figures." Figures 3, 4, 5, 7, and 9 have descenders; while 6 and 8 have ascenders. There are four styles of numerals:
Part I: Character Formats
Getting Started
Going with the Flow
Character Reference
Getting the Lead Out
Kern, Baby, Kern
Sweating the Small Stuff: Special Characters, White Space, and Glyphs
OpenType: The New Frontier in Font Technology
Part II: Paragraph Formats
Aligning Your Type
Paragraph Indents and Spacing
First Impressions: Creating Great Opening Paragraphs
Dont Fear the Hyphen
Mastering Tabs and Tables
Part III: Styles
Stylin with Paragraph and Character Styles
Mo Style
Part IV: Page Layout
Setting Up Your Document
Everything in Its Right Place: Using Grids
Text Wraps: The Good, the Bad, and the Ugly
Type Effects