These are some useful links to various published articles and books that include more guidance on the correct use of apostrophes and other punctuation.
A good summary of the rules
The Punctuation Guide
Excellent reference for all punctuation types, not just apostrophes
Good summary, including time expressions
A very thorough explanation of apostrophe use
A lengthy article with several references to the Apostrophe Protection Society
Eats, Shoots & Leaves
The Zero Tolerance Approach to Punctuation - Lynne Truss (Wikipedia link)
The Dreaded Apostrophe
Using the apostrophe correctly is easy - once you know the rule!