another wonderful Culture novel by Iain M. Banks.

The plot is quite complex (though I never felt overwhelmed) and is not easily summarized.
The world-building is excellent, the idea of a Shellworld brilliant.
Not only do we get another look at the Culture but we get introduced to several other civilizations of varying technological levels.

Banks examines (besides other ideas) in "Matter" the concept of meddling with/mentoring less advanced civilizations, a theme more superficially explored before in novels like "Use of Weapons" and "The Player of Games".

The plot starts deceptively simple in a "Inversions"-kind of way but expands rapidly to encompass characters from several species on different development levels and their interactions make for a complex and twisted plot.

Minor gripes: the start of the Novel felt a bit like "Inversions" (though that feeling was short-lived) and the end reminded me of "Consider Phlebas".

My advice:
If you are new to Iain M. Banks and the "Culture"-universe you may feel overwhelmed by the complexity in this novel. I suggest reading the "Culture"-novels in order of publishing (though I think the first three novels can be read in any order) and keeping this one for later.

If you are familiar with the other "Culture"-novels: read it, you won't be disappointed.