Back in … oh, quite a long time ago now; let's say about 2002 … I read Stephen King's fantastic On Writing. If you haven't read it, you certainly should.
One particular piece of advice stuck with me, but for completely the wrong reason. In fact, I'm kind of embarrassed to say it here, because just about everyone will probably look at me and say, "Duh. We could have told you that."
Anyway, the piece of advice was that you should cut 10% of a book's word count during revision. I read that, and I thought, Are you crazy? I can't do that. When I revise, I thought, my books get longer, not shorter. There's absolutely no way I could cut 10% on revision.
Obviously (I thought; feel free to laugh at me here), Stephen King's first drafts are very different from mine. He writes too much; I write too little. And, like all smugly ignorant wannabes, I left it at that.
Fast forward to this year, and I get feedback from an editor on my novel. She loved lots about it, she said, but it was too long. Could I cut it by 30% and send it back.
30%! The mind, briefly, boggled.
But, actually, for some reason, 30% seemed more doable than 10%. 30% says be ruthless. Be cruel. I set about rearranging scenes, removing certain unimportant side plots, figuring out different ways that other things could happen.
At the end of this attempt, I'd cut about 20% of the book. After that, I wasn't giving up. I gritted my teeth, remembered Stephen King's advice, and went through, word-by-word, line-by-line, paragraph-by-paragraph and deleted everything that didn't absolutely need to be there.
Now, I'm going to add a word about what 'absolutely need to be there' means. Because it's tempting when in mega-cutting mode to think that everything that either doesn't advance the plot or produce character development isn't absolutely necessary, but that isn't true. Books need colour as well as purely mechanistic progress, and those little bits of description, quirky dialogue, and incidental events are what make a book. That's not what I was cutting.
By the time I'd finished getting rid of all the really unnecessary stuff I had cut another 10% of the book, and I had realised something important: I use too many words in my writing. I repeat things that I don't need to repeat. I go on too long.
(And, incidentally, it made me realise another thing: if a scene or paragraph isn't working and I can't get it to work, it almost always needs to be cut; the book works better without it.)
I made a resolution at the end of that process: no matter how good I feel a book is, no matter how tight and how polished, my workflow will now always include a stage where I go through and cut 10% of the book before I let it go out the door, because no matter how much I think it doesn't need it, it does.
A final addendum: this book had been revised multiple times and read by several professional writers before it reached this particular editor's desk, and none of us had picked up on the bloat. We all thought it worked. And this is why books need editors, even (particularly!) self-published books. No matter how much work you put into it and no matter how many friends or other writers read and critique it, it needs professional editing to pick up the things the rest of you aren't seeing.