Nevada Barr's melding of natural beauty and human evil reaches a new height in the excellent Borderline, her 15th novel about park ranger Anna Pigeon. Set against the breathtaking beauty of Big Bend National Park in southwest Texas, Barr delivers a taut, suspense-filled plot that explores the hot-button issue of Mexican immigration and the post 9-11 closing of the Texas/Mexican border in the area.

Still recovering from the events in Winter Study, Anna is now on leave from the National Park Service. Anna fears the leave may be permanent, but she is never one to stay indoors so she and her husband, Paul, take a raft trip on the Rio Grande. The Chihuahuan Desert's beauty and the river's power may be the right medicine for Anna. The four college students also on the trip are congenial company. But the trip turns to tragedy when the raft is lost and the group finds a pregnant woman caught in a strainer between two boulders. They manage to pull her out of the river just before she dies. To save her baby, Anna performs an emergency C-section. After the group is rescued, an ambitious politician will use the tragedy for her own cause.

Borderline may be Barr's best novel to date. The author depicts the individuals affected by the border closing which, before 9-11, had operated with a more open-door policy. Yet never does Barr allow the issues she tackles to overwhelm the story. Barr keeps this series fresh by continuing to show new sides of her heroine. On leave, Anna finds that others in authority don't take her as seriously as when she was a working ranger. As usual, it's not wise to underestimate Anna.