Forgotten Truth, the third book in the Truth series, has little to do with the first two books in the series. The plot that dominated those two books was resolved at the end of the second book, and Hidden Truth has the characters pick up with their lives and then throws unrelated challenges at them. It is really more of a sequel than part of a compact series that follows one main plotline throughout.
In Forgotten Truth, Alissa, the main character of the Truth novels, is transported back in time approximately four hundred years to a time when the keep is thriving instead of abandoned. There, she meets Masters she has only heard about and encounters a much younger version of Lodesh, the not-quite-ghost she awakened in the second book.
Though it does not really continue the plot of First Truth and Hidden Truth, Forgotten Truth is still engaging and something that fans of the previous two novels will likely enjoy. Here, the reader gets to see Alissa mature some and become more confident in her abilities as both human and raku. Readers are also introduced to many new characters that they will, in a way already be familiar with from the first two books as Alissa meets the students, Keepers, and Masters of the past.
Cook does an admirable job weaving the plot, focusing mostly on Alissa and what she is learning and doing as she attempts both to find her way back to her time, but also giving time to Strell, Useless, and Lodesh as they try to figure out what happened to Alissa and how to get her back. The jumps from one time to another are never jarring, and they never leave the reader wondering too much, though they do leave some immediate things unresolved to keep interest. It is a difficult balance to maintain, and Cook does it well.
The returning characters in Forgotten Truth have mostly reached the point where they're rather stagnant and there is little development for any of them other than Alissa. They still have the depth that they had achieved in the prior novels, but they do not change much during the course of this one. However, there are several new characters introduced, and they are all also delightfully well-rounded, dynamic characters who are a lot of fun to get to know. It is the characters that Alissa meets in the past that keeps the book from becoming simply another "Lost in the wrong time" book.
Though the general plot has certainly been done before, Cook manages to keep it fresh and interesting through the characters and by tying all the events that Alissa experiences back into the stories and events from the previous two books. It isn't so much what happens that keeps it interesting, but more what the reader gets to see and who the reader gets to meet.
In many ways, Forgotten Truth is better and more exciting than certainly First Truth and to some extent Hidden Truth. Anyone who enjoyed the first two books in the series will like this one and newcomers to the series won't find themselves quite as lost as they would starting with the second book, though they will miss a lot of what is being referred to.