This is the latest in the Valdemar universe. It doesn't closely follow any of the existing books. It claims to be "volume 1 of the Collegium Chronicles", and Valdemar books almost always come in threes, so I assume it's the start of another trilogy.
If you like Valdemar books, you'll like this one for the usual reasons: the world is vibrantly real, Lackey has an amazing talent for making her characters matter emotionally to the reader, and it has blue-eyed talking horses. If you hate Valdemar books, you'll hate this one for the usual reasons: the hero is just too special, other characters are made to be absurdly nice or nasty, it's melodramatic, the plot borders on trite, and it has blue-eyed talking horses.
I like Valdemar books. 9 out of 10.
**** PLOT SUMMARY -- MASSIVE SPOILERS ****
Mags, our hero, is an orphan slave in a gem mine owned by a man named Cole Pieters. His situation is just about as bad as situations get; the kids have barely enough food, clothing, and shelter to survive, they are constantly worked almost, but not quite, to death, and they're controlled with emotional abuse as well as physical. Cole tries to keep Dallen, the Companion who Chooses Mags, out, but of course fails, and gets Herald Jakyr annoyed. This is Valdemar, after all, and crap like this isn't supposed to happen in Valdemar. It takes Mags a long time to realize that anyone will actually care about him or treat him decently. They dodge a nasty blizzard and make it to Haven, where the Herald's Collegium is just being built, physically and organizationally. Many of the Heralds are very uncomfortable about the idea of Trainees being taught in classes instead of directly by mentors, but the unprecedented number of trainees coming in leaves no choice. (Of course, an unusual number of Trainees portends bad times ahead for Valdemar.) Mags is so thrilled to have food, clothing, shelter, and the friendship of a Companion that he isn't willing to admit to himself that he's lonely and unhappy, but he is. Jakyr left him and ran off on urgent business; Mags has no friends. When he has a canceled class, he goes exploring and finds Bardic Trainee Lena crying in the garden over her dead pet rabbit. Lena is also very socially isolated, because she's the daughter of a very famous Bard and totally overwhelmed by people's expectations for her. Mags and Lena become friends, and Lena takes over tutoring Mags, who's way behind on his academic subjects and having a lot of trouble catching up. Then Lena introduces Mags to Bear, a Healer Trainee who doesn't have the Healing Gift but appears to have a Gift for tending and working with herbs. He's only a Trainee but everyone already considers him the master of herb lore.
Some odd representatives of an unnamed foreign kingdom come in as an embassy. Their bodyguards are nasty bullies. Three of them try to beat Mags up, but Mags has learned so much judo technique that he avoids them.
Midwinter Festival comes along and Mags' friends leave him alone. Lacking anything else to do, Mags goes into the city, where he sees a gem dealer trying to cheat a man in the marketplace. Mags points out that the "flawless" gem isn't, allowing the man to bargain for a fair price. The man, Soren, buys Mags lunch, and Mags tells him his story. Soren is horrified, and also finds Mags interesting. He invites Mags to his house over the holidays. When Mags goes to get permission from Caelen, the headmaster, to attend, Caelen sees whose address it is and explains that the fairly ordinary looking man in the market is actually an important adviser of the King. Mags goes to the party, befriends some of the kids, and gets drafted into an informal spy network, which also includes the King's Own Herald Nikolas. Mags starts spying on the strangers. The strangers' behavior is becoming erratic, as if they're haunted. Mags tracks down a conspiracy by some other trainees to combine Fetching and Far-Seeing gifts to persecute the strangers. One of them in particular is being driven mad. Bear gets drafted to give him something so he can sleep, and Mags and Lena go with him. They're convinced that the man really believes he's being watched, and Lena remembers a song about the vrondi, the magical guardians put in place by Vanyel. Nikolas believes that the man really is being watched by vrondi but they still don't know why.
Lena gets the idea that they should research the Guards' archives to see if they can find anything about Mags' origin. When they get their passes, Bear starts acting strangely about the archives. Then a huge blizzard descends on Haven. They're snowed in for three days, and then they realize that Bear is missing. Mags tries to find him with his Mindspeech and can't, but he does encounter the psychic trace of Something Nasty. They figure out that he's in the archives and realize that he's being held hostage by one of the strangers, an extremely dangerous fighter. They manage a daring rescue, with Mags and some other fighters distracting the man while others spirit Bear out. When he realizes that his hostage has been stolen, the man suicides. Bear reveals that the reason he was sneaking into the archives is that he's been betrothed to someone he doesn't want to marry. He wanted to find proof of the rumors of something bad in the girl's family.
The book ends right here, without us or Mags actually finding out about his origin, with the foreigners escaping from the palace and nothing more specific about the shadowy danger that will become fully manifest in the next book.