Mizushima, Margaret. Timber Creek K-9 mysteries.

"Deputy Mattie Cobb and her sister, Julia, travel to Mexico to visit their mother, but when they arrive, they discover that she and her husband have vanished without a trace. Back in Timber Creek, Mattie finds a chilling note on her front door telling her to look for "him" among the standing dead up in the high country. The sheriff's department springs into action and sends a team to the mountains, where Mattie's K-9 partner, Robo, makes a grisly discovery--a body tied to a dead pine tree. Mattie is shocked when she realizes she knows the dead man. And then another note arrives, warning that Mattie's mother is in desperate straits. In a last-ditch gambit, Mattie must go deep undercover into a killer's lair to save her mother--or die trying