Funny note about Mercedes Lackey's The Hills Have Spies: The description on the book jacket has absolutely nothing to do with the actual plot. Indeed, the description mentions Mags's son Justyn and Hawkbrothers. Then you read and find out it involves Mags's son Perry and a trip to the Pelegir Hills. Indeed, none of the children is named Justyn.
So, anyway, at the outset, we learn that Mags and Amily have four children that they're raising along with the current King's brood. While all the children are being raised to be escape artists, Perry is in the running to be the next Herald Spy, unless he doesn't get chosen. Perry has Amily's gift of Animal Mindspeech, which while good, doesn't allow him some of the fun things his father can do.
After everyone's been properly introduced and roles established, Perry and Mags end up taking a father and son trip out to the Western edges of Valdemar to investigate something a semi-retired Herald is noticing with odd disappearances among the people out that way.
Posing as traders, they get out to Herald Arville, where upon Perry promptly gets chosen by the neuter kyree Larral. They help the village form a mining pact for the garnets in the ground, then wind up finding out about the kidnappings happening from an inn in the forest.
Perry, with Larral's encouragement, ends up leaving his father while he sleeps to scout out where the people are being kidnapped to, which leads to a city in the hills. (We're skipping over various species of intelligent animals they encounter, like raven Bondbirds and dyheli, and eventually firebirds.) Said city is controlled by someone using blood magic and mind magic to control a mercenary company and those they kidnap. While it's not mentioned if this is another incarnation of Ma'ar, it could be, although Ma'ar's incarnations usually were more subtle in their intrigues.
Perry disguises himself as a dog trainer and takes over the kennel.
What follows is cloak and dagger type things, as father and son try to outwit and outlast the Master and figure out how to come away from this alive.
While parts of this could use more fleshing out, it's a pretty good entry in the larger world that it's set within. If we're going to continue to be stuck with Mags, adding in others will help.