I have a photo that an author would like to publish in a book. I sent it at a resolution of 300 p/in. She would like it at the highest resolution possible. How do I find or determine that value? The photo was taken with a Nikon 7000.
The ppi setting is only a tag in the file metadata, it does not change the image in any way. It may be used by some printing applications to determine a default print size, but is generally ignored. The ppi box in the LR Export dialog will accept any number from 1 to 65,000, but what really determines resolution is the number of pixels in the image. If you are exporting all the original pixels from the camera less any lost to aesthetic or aspect ratio crops, that is the highest native resolution possible. Resolution can be artificially increased by using the Resize option in Export so that LR will create and add in new pixels - whether or not this causes visible artifacts is largely dependent on the amount of enlargement.
Since your author friend is probably also unaware of the irrelevance of the ppi tag, send her all your native pixels and set an impressively high number for the tag - like 65,000