Perry, the oldest son of Mags and Amily is well on his way through training as a spy for the kingdom. While Perry doesn't have the Mindspeak gift that his father boasts, he does have something similar, Animal Mindspeak. Where Mags can read the thoughts of people and talk directly to their minds, Perry does that with animals, and its a gift that will come in handy during his budding career as a spy.
While Perry's training in all-things spycraft is going exceptionally well, there is one aspect that seems to be missing, and both Mags and Amily are very conscious of it. Perry has yet to be Chosen by a Companion, and without that he can never be a Herald like many of the people Perry interacts with. Is the time simply not right, or does Perry's life lead down a different path? While they aren't pushing for Perry to become Chosen, Mags and Amily know that the fact that a Companion hasn't presented itself to Perry yet has to be weighing on the boy.
When a Herald on the border of Valdemar sends word to the King and Queen that he has noticed an odd pattern of disappearances in the area, Mags is approached. Instead of sending a contingent of guards to the rural area, the King decides to ask Mags to unofficially look into the matter so that the situation can be properly assessed and the right actions be taken. Mags and Amily decide that this is the perfect opportunity for Perry to get some field-training in and both Perry and his father hit the trail posing as traders in the hopes of learning exactly what is happening on the roads near the Pelagirs Forest.
As Perry and Mags' journey progresses, they will befriend Larral, a strange and incredibly intelligent wolf-like creature. When Larral declares that he chooses Perry, everyone around them is stunned. While the relationship isn't exactly that of a Herald and his horse Companion, it's quickly clear that both Perry and Larral benefit from their growing connection in ways similar to the one a Herald gets when Chosen, and Perry will need every advantage to survive his experiences. Perry ends up getting separated from Mags, and the boy will have to rely heavily on his training, his Animal Mindspeak, and his new connection to Larral in order to make it out of this trip in one piece.
The Hills Have Spies is not just a great continuation of the massive Valdemar series, but it works pretty well as a entry point into that universe. Newcomers to the series might have a bit of a learning curve ahead of them as the book doesn't go into too much detail about Heralds, their Companions, and what it means to be Chosen, but given Perry's particular situation, those details aren't all that necessary. What the reader does need to know is given out at the right pace so you never feel lost because you don't know terms, nor do you feel bombarded with facts that are irrelevant to the story.
Mercedes Lackey is quickly climbing the ranks in my personal list of favorite fantasy authors, and I think I am going to have to delve much more into her Valdemar series before all is said and done.