Seeing as a large part of interactions with the Tyrell family are seen through Cersei's narrative, and given that Cersei is somewhat of an unreliable narrator (read: malevolent bitch,) it's hard to get a sense of whether the Tyrells really have malign intentions. Cersei seems to suspect early on that they are trying to usurp the throne, but is there good evidence that this is actually the case? Do we know what the Tyrells' long-term plans are, and why they came to King's Landing?
In the TV series, it's a lot clearer that the Tyrells are manipulative; the interactions between Margaery and Olenna in particular. So my question is directed at the books, where these interactions don't take place and the machinations of House Tyrell are far subtler.