Take some time to research a good synopsis, how to write one, what agents are looking for, and how writers feel about them, and you'll find so many different, often contradictory opinions, that you might as well just figure out the best way for YOU to write your synopsis. In the past, I mentioned that there's no such thing as a "standard" synopsis. It's a total lie. Some places tell you the standard is one page, single-spaced, no paragraph indent. Others will say that it MUST be double-spaced, from 1-8 pages, following ALL of the plot points, side plots, main and ancillary characters. And pretty much anything in-between.
What is standard? There's a 90% chance you MUST have A synopsis in one form or another. Beyond your basic query letter. Why? Some agents still require them. Many editors require them. And it's hard to find a novel-writing contest that doesn't require at least a minimal synopsis.
"But," you say, "I'm self-publishing. Do I need a synopsis?"
Maybe. I don't know. I've never looked into self-publishing. But if I WERE to go that direction, I would write a synopsis. And it would be the best darn synopsis I'd ever written for that book. Why? Promotions! Whether or not I spoil the ending in the synopsis, it's a great tool, especially at a shorter length, to use for promotions.
And answer me this question: What's your book about? I'm a potential reader, you're a self-pubbed author drumming up sales. You caught me with the book's cover. Awesome! Now you have two minutes. TWO MINUTES to sell this book before I lose interest.
"That's not fair," you insist.
No, it's not. But that's life. The average American has a 30 second attention span. The average reader may stretch it out to two minutes, especially if they're properly hooked.
"But I spoke to readers for ten minutes or MORE!"
Doesn't matter. Polite people will hang out and talk for a long time, especially if you're friendly. But in sales, especially when browsing, will make up their mind VERY quickly about whether or not they'll make the purchase.
"Okay, Giles. What the eff does this have to do with a synopsis?"
What's your book about?!? Have you taken the time to lay it out from beginning to end, picked out the MOST IMPORTANT parts, and then crafted them into your pitch? Yes? Then you may be fine. If not, you need to figure that out, and one of the easiest ways to do so is to write a synopsis.
Of course, this is all highly biased opinion, but that's my two cents. For the record, writing my synopsis is by far the most painful part of the experience for me. But to move forward, it's absolutely necessary.