Glucksberg argues for what is called the "property attribution model" of metaphorical comprehension, which holds that metaphors are essentially akin to statements like "Trieste is no Vienna", where "Vienna" is not the name of a city, but stands for some exemplary category that characterizes Vienna (in this case, something like being a cosmopolitan city). This contrasts with the traditional picture (advocated by Grice and Searle, for example), that comprehending a metaphor involves first grasping its literal meaning, which is usually semantically anomalous in some way (so "Trieste is no Vienna" would be read as absurdly obviously true), which then triggers a metaphorical interpretation that precedes by applying general conversational principles (informativeness, e.g.) to derive the metaphorical meaning (Trieste is not a cosmopolitan city).
Glucksberg gives convincing evidence (involving speed of comprehension tests) against the idea that listeners have to compute the literal meaning of a sentence before they can give it a metaphorical interpretation. Though I confess that it's not at all clear to me what "understanding" or "comprehension" of either a literal sentence or a metaphorical one is supposed to amount to; as far as I can tell it's when the test subjects indicate that they understand, rather than actually successfully explaining it (what would it even BE to show that you had understood a literal sentence or a metaphor? Obviously, it depends on the circumstances). So the tests seem to measure the feeling of understanding rather than understanding itself, which, as Wittgenstein is at pains to point out in the Investigations, is not anything like a unified phenomenon or simple inner state.
And throughout the discussions in this book of which metaphors are interpretable and which are not, I found myself disagreeing with some of Glucksberg's assessments. For example, Glucksberg says that metaphorical comparisons are "nonreversible": "The clearest examples of nonreversibility are provided by assertions that become anomalous when reversed. For example, ALCOHOL IS LIKE A CRUTCH becomes uninterpretable when reversed (A CRUTCH IS LIKE ALCOHOL)" (p.32). Really? A crutch is like alcohol in that it numbs (part of) you, it is like alcohol in that once you start using it all the time, you don't want to stop, or that it's like alcohol in that it's a good way to start a conversation at a party. I'm sure there are other ways in which a crutch is like alcohol.
There is a lot of interesting material in the chapter on idioms, which thoroughly debunks the idea that idioms are just "long words" with no syntactic or semantic complexity. For example, an idiom like "skating on thin ice" is quasi-compositional: "jumping on thin ice" and "skating on thick ice" seem to have different but systematically related meanings, though some work would be required to make sense of either "skating on thin water" or "skating under thin ice".