When does a photo album become a book?

[From Inside on Flickr - Photo Sharing!]

This image from flickr by a commercial photographer working in the states has me thinking about what differentiates a photo album from a book? More specifically, a 'Photo Album' from a 'Monograph'. Is it just me or does the phrase 'Photo album' have connotations of a highly personalised history, family or otherwise, whereas a 'Monograph' has slightly more serious undertones, with a whiff of Academia?

Which of these values, defines one over the other, can one be both?

• Photo corners?
• Distribution?
• Content?
• Production Costs?
• Production Values?

Time to make another I guess?