Just a quickie post, but have you ever realized that Amazon.com is the default book link of the web? The vast majority of instances when anyone links a book title online, they're pointing to Amazon. Two fairly obvious reasons why:
1. Their selection is so broad, they almost certainly have a page for the book you're referring to.
2. Meta-info and community book reviews make their pages feel useful for readers. It's almost like it's not even an online store. Almost!
Wouldn't it be amazing to become the default music link (iTunes)? The default reference link (Wikipedia)? Are there other content-type or general category defaults like these?