In the previous chapter, wefocused on the network layer, which deals with transporting gobs of data from one host to another. The main security concerns at that layer were confidentiality, authenticity and integrity of the data transmitted.
In contrast, the presentation layer deals with the structure of the data transmitted. When you get to this layer, you have already decided that you want to talk to the peer. You've either made sure that data is protected (by whatever means) from eaves-dropping and modification; or you have decided that you don't care. For instance, much of the world wide web is public information anyway, so confidentiality is not an issue for that data. Neither is integrity in many cases, because there's little value for an attacker in trying to fake e.g. Saturday's soccer score.
Instead, the main question at the presentation layer is whether your application can handle malformed data. Robust applications will respond to malformed data either by giving their best trying to come up with a reply (even if it's meaningless), or simply drop the request (and in the case of connection oriented protocols, the entire connection with it).