Thanks for reminding me about this one! Another octopus themed book that was quite fun is Children of Ruin, the sequal to Children of Time (post-space-apocalypse, a hubristic uplift experiment on monkeys accidentally creates a species of sapient spider over the course of ten thousand years, while the few surviving humans dip in and out of cryo to deal with their own problems).

A lot of work went into extrapolating a non-human mentality for the animals (the spiders in the first, octopuses and mystery goo in the second).