Fourth book in a very imaginative and fascinating series.
The sequence is: Book One, City of Pearl
Book Two, Crossing the Line
Book Three, The World Before
Book Four, Matriarch
Book Five, Ally
Due in April 2008: book 6, Judge
Works best when read in this sequence. Very rare to find an SF series where the aliens are both plausible and genuinely different from humans. Also a good take on the ecological aspects of relations between different species. If you like David Brin's two "Uplift" trilogies ("Sundiver" et. seq.) then you will very probably like this, and vice versa.