Even i faced this question and here is what i think now.
Add to an existing hub only if the new information adds any value to the existing hub. If the existing hub is already 500+ words or has enough pictures etc to look complete, it is better to create a fresh hub.
It is easier to promote a new hub. Also, it is easier for the readers of your hubs to reach the specific info they are looking for rather than searching for it in lengthy hubs.
I think it would be better if you create a new hub for the book you want to write about and add a link to that hub in your existing hub.
Remember Hubpages supports Interlinking of hubs, so always add links to relevant hubs (yours or others) to your hub.
This keeps things neat and also helps you gain more 'Hubkarma'