When I give an app access to my Evernote account, can I say, "only access XYZ notebook" or "you only have the right to write, but not to read, my notes"?
It seams Evernote at the time of writing has no support for OATH like many others do. :(
The only way at the moment to give a 3rd party access to your notes is to give it your password. If you give your password to a 3rd party they will have the same access rights to your Evernote as you have. This is true for any other service.
UPDATE: I was wrong! I could link my Evernote account to Greplin without giving them my password. In Evernote it appeared under Settings -> Authorized services. So Evernote does support a form of OATH or can give 3rd parties access to their API without revealing your password to them.