HI David,
There is an ongoing project to achieve “granular permissions”. Coda has divided the project into 4 phases. The first two has been implemented; Coda has recently announced that the availability of the 3rd phase is imminent.
Phase 2 allows you to share pages from a doc, but people can still access the source doc. Phase 3 will prevent people from doing that. For more details please see the below link.
This is one of the most frequent requests, but it is also extremely difficult to implement. E.g. what to do with formulas using other areas in the source doc, how to handle relationship (look up) tables etc.