This is more of a memoir about experiences with octopuses than a popular science book about them, despite where Waterstones shelved it (at least in Brussels). It can get a bit woolly at times, since Sy Montgomery seems pretty preoccupied with how she experiences her meetings with octopuses, and what it does to her soul, but there are some interesting bits and I did enjoy reading about the individual personalities of Athena, Kali, Karma and Octavia.
Sometimes I’m not sure at all how well founded her speculations are — for example, she mentions a theory about an octopus taking a dislike to someone because it could taste the fact that she was on medication. Theoretically, I guess that’s possible, but practically, I’m not sure it’s true — or how on earth we could possibly know what an octopus senses, and whether it likes or dislikes what it finds.
Peter Godfrey-Smith’s Other Minds is the better book about octopuses, I think, and goes more into a philosophical examination of their consciousness. This is mostly about Sy Montgomery — which might be your thing, but isn’t so much mine. The more I think about it, the less I rate it.