A couple of weeks ago, I went to a meeting of the Northern California Chapter of the American Society of Media Photographers – asmp NorCal, to you. In one of the handouts, was an article on Photo Apps by Lee Foster.
Using a book he wrote on where and how – presumably, I haven't seen the book – to photograph in San Francisco, Foster goes through the numbers on the book and an app.

The book retails for $14.95, but typically sells through Amazon at a hefty 55% discount. So, his publisher gets $6.73 of which he gets 15% – that is $1.01. He is now converting the book to an app which will sell for $1.99. In the app world, the author typically gets 30%, the developer gets 30%, and – in the case of the iPhone – Apple gets 30%. The remaining 10% is for overhead and – I guess – gets beamed up to some unknown place. So, for the sale of two apps at $1.99, Foster gets $1.19.

He thinks that there is a better chance of two people paying $1.99 for an app than one person popping for $14.95 for the book. I think he is right. Even a casual visitor to San Francisco would probably be willing to pay $1.99 but, paying $14.95 for the book that you would then have to pack and carry to be of any use, is much more problematic.

And the app could be much better. In Foster's case, the book has about 70 photos, but the app has 100 photos. And he expects the next app to have 500 photos. It could also have interactive maps and videos.