The trilogy comprises Song Quest (1999), Crystal Mask (2001), and Dark Quetzal (2003) and follows the tales of The Echorium - the singers located on the Isle of Echoes.
Set 20 years after Song Quest, Crystal Mask introduces Rialle's son Renn as a novice at the Echorium, who must travel overseas when the arrival of Shaiala, a wild girl who claims to have been raised by centaurs, casts doubt on the long-held belief that Frazhin is dead.
Between them, they discover that Frazhin, with the help of his lover Yashra, has been kidnapping children and enslaving Half Creatures in an attempt to build a khiz palace of the Songs to rival the Echorium.
By combining Renn's songs with Shaiala's centaur kicks, the khiz is defeated and Yashra captured, although Frazhin disappears, presumed drowned.
Eleven years after Crystal Mask, Kyarra, a gifted young novice, is horrified to learn from her friend Caell's discussion with the merlee that her parents were not Singers, and she falls into a trap and is kidnapped.
Meanwhile, Night Plume, a dark-coloured quetzal under Frazhin's control, suddenly finds himself free after being sung to by a captured Rialle.
All Half Creatures are protected under treaty from being hunted or enslaved, and while not all can speak aloud, they can communicate using Wild Speech with each other, and with certain Singer novices.
Merlee have the power to control the weather on the ocean, creating storms, mists or clear days depending on their mood and their relationship with those on the ships currently travelling.
Their children are born from eggs in caverns near the Fall of Clouds, and all naga obsessively collect glittering objects, generally called "sparklies".
Centaurs can use these herdstones to bend light around themselves, rendering the herd invisible from the outside, and they used this ability to hide their existence from humans until the events of Crystal Mask.
Although bluestone is only found on the Isle and is considered very valuable, it is occasionally given away as a "trust gift" to those who make treaties with Singers, so that they can be sung to over a distance if the need arises.
The Isle of Echoes is a small island made entirely of bluestone and situated in the Western Sea, ten days sail from Silvertown and thirty from Southport on the Purple Plains.
It is located in the centre of the Isle of Echoes and includes housing for Singers, novices and orderlies, classrooms, treatment cells, offices and the Pentangle.
At the end of their schooling, only those with the best voices are accepted as Singers, while the others are given a mild form of Yehn to make them forget the Songs and then are trained as orderlies.
Orderlies and Singers alike must work for a certain number of years to repay the debt for their schooling but are free to leave The Echorium, although many choose to stay.
It is one of the main settings for Song Quest, as the base for both Frazhin, who acts as the khizpriest and head of the area's religion, and the merlee hunters wanted by the Singer delegation.
The inhabitants of the region - known as Karchholders - are known as ferocious warriors, and wear the finger bones of their dead enemies in their braided hair to show prowess in battle.
To the South of Silvertown, the Purple Plains is a large, mostly empty region occupied mainly by nomadic tribes called the Horselords.
Although much of the region is uninhabited, some large towns on the rivers aid the Great South trade route, including Southport and Rivermeet.
The area is home to both centaurs and nagas and is a source of headstones in the Dancing Canyons and khiz crystal, in the Sunless Valley which can only be reached through underwater tunnels or through the Pass of Silence, which is prone to avalanches.
The Forest is the main setting for Dark Quetzal and is the home of Night Plume's flock in Frazhin's Starmaker temple.