What puts you off a book immediately?

No quotation marks when people are speaking.

Chick lit
Woke shite

Not the author's fault, really, but last year I read a book set in the UK in 2019 that had a 'flash forward' to one year later, where the characters had a huge house party ... which would have been bang in the middle of a strict lockdown as it turned out.

Moral of the story - don't write about the very near future - you never know what might happen!

I was recently put off a psychological thriller because the author described a woman as having "tresses". Just don't, it's not Rapunzel!

I've just read a novel where the protagonist (who everyone simply loves and adores) throws a 40th birthday party in her garden for 100 people. She is irritated when someone comes to the door who she assumes is going to be a neighbour complaining about the noise, even though she put notes through everyone's doors warning them the party would go on till 3am. Selfish cow I thought.

Going back to OP, I too dislike epistolary bits to move the plot along, rather than actually writing some plot. I read a lesser-known Wilkie Collins recently and there were too many letters explaining motivation and next move, eg “Dear Clarissa, I must tell you immediately of my journey to Broadstairs to meet with the mysterious captain of whom we yet know so little. Well, it turns out he is Captain Pertwee whose ship has sailed from…… “

Stupid younique names

Obvious 'chick lit' covers (bright colours, stick girls with shopping bags etc).

I can't stand badly written books , i know straight away and will usually ditch it, plenty of well written books to read so don't waste my time !
A child character , too young to speak but given script anyway in stupid baby talk, i just started The serial killers wife , both badly written and baby talk - waste of 99p on amazon !
Pretentious character names and also as someone mentioned earlier using fashionable names of the moment on older characters.
Anything with the slightest whiff of chic lit.

Books written in the format of diary entries or letters. Yuk.

Ahhhhh, but therein might lie a clue , @WilliamofBaskerville ! I think it’s in A Murder is Announced (I may well be incorrect) where a character says something like, “Yes, Margery, I just can’t take it any more.” And then later says, “Yes, Marjoram, I just….” And there’s the reader, if they notice at all, thinking it’s a misprint. But no!!!! (Sorry, I’ve somewhat revealed the murderer now….)

The modern trend of writing in the present tense as has been mentioned here. It's irritating in the extreme and just so wrong.

Books where the characters address each other by their first names all the time:

'Margery, I've got to tell you something... I'm leaving'
'Oh, Peter, surely not?'
Yes, Margery, I can't take it any more'
Peter, you can't, what will I do?'
I don't know, Margery, you just have to get by...'

I love, love, love Agatha Christie novels - I know she's not renowned for her writing style - but she is incredibly guilty of this, and once I noticed it, it really jarred. Still reread them loads though!

I'll add in any that are written by Sophie Kinsella, as her bloody shopaholic heroine was toe-curlingly cringey.

Did someone here say Apple Tree Yard was awful?? I loved that book.

I dislike any novel where a character lies prone on a sofa. Prone means face down!

I hate the series books where huge amounts of the book are spent going over old ground, i.e. explaining what happened or character introductions that had already been done in the earlier books of the series. Yes, I know they do it for people who've not read earlier books of the series, but it's very annoying for those who have.

The latest Wilbur Smith was terrible for that. Constantly going over old ground and it just ruins the flow of the book.

Combining several bad tropes, I read a book not long ago where the heroine's small petite curvy figure with tumbling curls (that she battled with) turned into her tall straight boy-like body within two pages.

Any book cover that says "by Martin Amis" on the cover

That's entirely fair.

I recently gave up on a book because the whole thing was written as a transcript of recorded interviews with the characters. And each character was given full name in their first quote only, the rest were initials, so difficult to remember
I hate it when interviewers do this in magazine articles, even worse in a full length novel.

Any book cover that says "by Martin Amis" on the cover
The word "girl" in the title (with a few honourable exceptions)
A cover showing cupcakes, prosecco or high heels.
Any book where the blurb claims it is "hilarious" or "laugh out loud".

Ant sort of graphic rape scene.

Books by middle aged blokes about a middle aged bloke who is constantly fighting off women fifteen years younger than him.

Loads of characters introduced at the beginning - I will forget them and read the rest of the book in a state of befuddlement.

Poetry

Anything where women forgive shitty behaviour by men because they luffs them

Creepy descriptions of women. Particularly if in odd outfits - she looked sexy in her blue silk blouse, the heels of her stilettos made her long smooth legs appear even longer, her breasts swelled at the neck of her draw-string top

Dotty Harmer........having a mane of tumbling curls would be my no.1 bugbear....