This is the last of the Culture books by Ian Banks. The Culture is a rarity in sci fi these days, a utopian future in which the good guys are not only powerful, but they kick ass as well. The thumbnail version of what they are is anarchistic but also highly liberal, but governed by super ethical AI's infinitely more intelligent than humans. Indeed, many criticize the Culture because they consider humans as little more than Pets. If by 'pet's' you mean that biological and technological intelligences are both granted equal rights, and in a post scarcity society participation is governed by willingness but also ability, then I guess that makes biologicals 'pets'.
Except that in each of the Culture books, Banks manages to give the biological protagonist significant meaning. It answers the question of what might happen when we truly do have AI's in the best of all possible worlds, but does so in a realistic format. The AI's and the humans are believable, as are the villains. Indeed, one of the things that sets Bank's works apart from...for example, militaristic technophile Sci Fi (which is a genre I still like) the bad guys are not parodies but extremely complex characters. Granted, they're not usually "magneto" level anti-heroes but they have a certain depth to them.
I also greatly like the Ship Minds, the essentially rulers of the Culture, who spit in the eye of the Prime Directive and definitely meddle in the affairs of other species in a benevolent way, while doing so with both ethics and in a manner to allow them to evolve as meaningfully disparate. These ships have names that are snarky jokes but their personalities also show compassion and intelligence far beyond that of all but a relatively small slice of humanity. They're also really broadly speaking just 'cool' and Banks does what I might call a literary 'superman' by carving out a meaningful story despite a society that has technology more advanced than almost any sci fi setting I've read short of 'godlike' as in 'so advanced we don't even bother to explain it.'
The Hydrogen Sonata is about a culture that was nearly part of The Culture, but instead of joining the Culture is now choosing to exit the galactic scene by a dimensional retirement method known as 'Subliming.' The Gzilt (the race in question) are unique in that their 'bible' is actually 100% compatible with science from their stone age through hyperspace bypasses. However, a month before they are going to Sublime, the race that made their Book of Truth, is about to reveal that there are one or two things in it that were lies...and the leader of the Gzilt doesn't want this to get out. So they disintegrate the messenger. The Culture decides that they want the truth protected at the least, and so the story involves a series of murders, chases and intrigues as the lost secrets are sought out across an interstellar stage.
I like these books and I liked this book in particular. Bank's death is tragic on its own but doubly tragic in that there will be no more of these books, which are an excellent mirror of a future that 'might be' without resulting to an unrealistic 'polyanna' feeling that sometimes occurs in other tech heavy utopian futures. Banks had an excellent writing style and his characters, both bio and techno are very interesting.
In short, I highly recommend reading this book. The nice thing about the Culture books is that, like Terry Pratechet, one can read any of them independently without any particular order. Get it as you can.