I just finished reading Matter. No reading-matter jokes please.
There was quite a large fantasy element to this, as much of the plot revolves around a civilisation barely into gunpowder and steam engine technology, and then only because they've been fed it by a more advanced race. I don't mind that, fantasy-in-SF was used to great effect in Implied Spaces, but I felt it made much of this novel a little on the slow side. On the upside, this also forced the fleshing out of the different levels of Involved civilisations in the Culture universe, in fact showcasing one of each of four distinct levels of races of sentients, each one a mentor for the race below, with the Culture obviously on top.
This book introduced the shell-world concept, which is essentially a constructed planet formed of many concentric shells, although existing in more spacial dimensions than normal space. Each shell level can hold a whole different race and environment tailored for them. A very cool and brilliantly detailed technology.
The ending was a little rushed, I felt. I think it would have been good to have focussed more on events in the second half of the novel than the low-tech political shenanigans in the first half.
Still, a good read. Iain M. Banks rarely disappoints.