When reviewing first answers or late answers, I fairly often come across answers along the following format:

or

A proof of this can be found in book X.

A couple of examples of this is user31266's answer to this question and mFriction's answer to this question.

While these are technically an answer, I feel like they are not really relevant to this site, because

1. the material in the link or book may be extremely advanced or extremely lengthy,
3. in the case of books, the OP may not have access to the book in question.

So my question is, what is the usual procedure for these kind of answers? Should they be deleted or flagged, should I leave a comment, asking them to put some of the material they reference into the actual answer, or should they just be left as is?

A related question was discussed here, but I feel it doesn't quite answer my question.