The link field can be used to open other applications. "iBooks", "Kindle", "Stanza", "Google Books", or "Nook" can be selected to open the respective application when pressed in non-edit mode.
Unfortunately, these apps don't provide a way to open to a certain book or query for the books contained within the app. I've contacted the developers with this request, but no one has responded to me. I will be happy to implement it on my end if it ever becomes available. Perhaps if enough users contact them it will become possible!
Alternatively, turn on "Manual" to enter a URL. As an example, "http://www.chiisai.com" entered into the link field will open safari to that URL when pressed in non-edit mode. You can also enter any link which will open an application on an iOS device. Goodreader provides the ability to copy a link that will open to a specific document. Pasting the link into this field will open to the associated document when pressed in non-edit mode.
If an invalid URL is entered into this field, it will be treated a a custom field.
The "Link" field can also be renamed under Settings.