This is an omnibus edition of the middle quartet of the Tortall series. You don't have to have read the first quartet to understand this one however. It stands quite well on its own. Our heroine is a young girl who has Wild Magic and can talk to most of the animals in her land. The Immortals of the title are beings that were forced from the land by the human mages. This series tells the story of the Immortals' fight to come back to Tortall and the humans fight to keep them out.
This is a young adult series, but is great fun for adults as well. Pierce is great at world building and describes everything so the reader can see exactly what she's talking about. Her characters are well-drawn and grow throughout the series - something sometimes lacking from YA material. This series would be especially good for young girls, peopled as it is with a great number of strong female characters.