The Hydrogen Sonata is the story of the Gzilt civilisation's last days - their final breath, as it were, before they ascend to a higher plane of existence, as a collective, via a process known as Subliming.

We witness the build-up to this event from a few perspectives, our primary protagonist though is a Gzilt musician called Vyr Cossant.
She plans on using the time she has left before the Sublime to attempt to master a notoriously difficult-to-play piece of music known as The Hydrogen Sonata.
She is interrupted in this endeavour by a huge last-minute political upheaval.
Vyr, alongside (and with the assistance of) the Culture ship "Mistake Not...", becomes embroiled in a quest to find the oldest living person, who apparently has answers about the Gzilt's unique and apparently mysterious place in the Galaxy.
All the while, other galactic species circle overhead, ready to pick over the bones of the civilization once her and the rest of the Gzilt move on.

With a story like this, the themes and metaphors are somewhat obvious, but trust me - the execution is masterfully performed.
There is humour, sex, religious skepticism, simulation theory, a ship mind group chat, and a few brilliant drops of silly nonsense that all weave into the action, painting a very well rounded and very "Banksian" reflection of an all-too-familiar species.
More importantly, however, it tackles the themes and concepts of long lives, forgotten memories, the legacy one leaves behind, coming to terms with one's own death, moving on, art... and leaving art feeling unfinished.

This was Banks' last Culture novel. At the time of writing this he was battling cancer.
He knew this was his farewell, and what a farewell it makes.
It is utterly brilliant and a fitting end to this incredible series.

An easy 5 Stars.

I think this book superbly bookends the Culture series, as almost the opposite to its first entry; Consider Phlebas.
Not only narratively, and perspective-wise, but also when comparing and contrasting the two, you see just how much Banks improved when it came to structuring his stories, because (however fantastic) Consider Phlebas has it's shortcomings, but this was flawless and truly fantastic.

I held off on reading The Hydrogen Sonata for a few years, not being quite ready to say goodbye to this universe, and having now done so, I suppose I'm still not ready to leave it behind, but alas.
I will without a doubt be doing a series reread many times in the future.

P.S.
Amazon... Please stay away. I don't like or trust you enough to have any faith in your interpretation of the Culture.