One of the big themes in A Plague of Wizards is isolation.  Many characters have to deal with some form of isolation.  Senta is isolated from everyone that she knows and loves for a long time, and even when she returns, she feels an emotional distance from them.  Esther is isolated in that she is the only lizzie on a continent of humans, and likewise, Terra is the sole human living in a city-state of lizzies.  Both Baxter and Saba feel isolated because of the broken relationships in which each finds himself.