BETTER than both Gone Girl and Before I Go To Sleep! A thoroughly deserved five stars, this has been one of my favourite books of this year.

In this world, there are two kinds of people, those who can store two days of memories (duos) and one day of memory (monos). To keep track of their lives, they maintain daily diaries. When Mark, a Duo, weds Claire, a Mono, they have a happy life until a woman, who turns out to be Mark's mistress, ends up dead. Who can you believe when you can't remember anything? Wickedly clever and inventive

This novel starts with an unusual premise, that people are divided into two categories, Monos who possess a 24-hour memory, and Duos who can remember everything for 48 hours. Rather than class, gender or race, society revolves around the differences between Monos and Duos, with Duos being seen as more intelligent and generally superior to Monos. From this come rules restricting what grades of job a Mono can apply for or aspire to.

People remember 'facts' by writing them in their iDiaries every night before they can forget them. These facts seem to be memorised very easily and that is how people function and understand their lives.

This intriguing fantasy is segued with a thriller plot: Yesterday is a murder mystery. It is also an exploration of memory and how we understand the world, the narratives we tell ourselves, and an examination of love and hate.

So far so fascinating. Yet I am awarding this book only 3 stars because of the way it is written. The narrators of the book are the four main characters, yet I found their voices to be much the same voice (with the exception of one very sweary account). I am of course speaking of my personal taste but I found the writing clunky, and overly laden with adjectives - for example this is what one character says to another:
'The figure was moving about with an ethereal, feline grace that could only be feminine. Yet she was also circling the room in a slightly agitated manner; she resembled a hungry, desperate panther. I squinted; she was clad from head to toe in midnight black. A sinuous sable scarf concealed most of her face.' This sort of overwriting partly spoils the book for me: it slows action, defuses tension and, well, irritates me.

Yet still I awarded the book 3 stars. It is a great story.

So I was very drawn to the concept here: an alternative present where society is divided into those whose memory goes back a single day (Monos) and those who can remember two days (Duos) but I'm afraid I really disliked the style of writing. It was, in my opinion, desperately over-written where it seemed to take pages and pages, even whole chapters, to thoroughly flog various dead horses. Yes, I get it. You want revenge. Do we need a fifteenth chapter devoted to you announcing it? Plus some of the language just wasn't my sort of thing. 'Silence coated my lips'. What's wrong with just not speaking? Of course some people won't mind flowery prose and if not, the twists aplenty in this murder mystery might be enough to sustain their interest.