I would suggest starting with the standard Time quartet, like everyone else, because Meet the Austins is kind of . . . cloying. But the third in the Austin quartet (A Ring of Endless Light) is really quite a good book.

Now that I think of it, I'm pretty much always a fan of the sequels. Camilla is OK, but I liked A Live Coal in the Sea better. Ditto The Small Rain (OK) and A Severed Wasp, although, again, it depends on whether you're aiming to read her straight fiction or the SF-y stuff. (And how much religion you can stomach.)