Synopsis

In addition to a query letter, agents/publishers will probably also ask you for a synopsis, so you might as well start on it after your first draft.

I won't bog you down with too many details on a good synopsis, because there's already so much info out there, and a lot of it conflicts with each other. I will only say this: A good synopsis should be short. (2-3 pages if it is double spaced and uses a twelve size font) Also, it should be entertaining. You shouldn't write it like a summary on Wikipedia.

If an agent likes your query letter enough to continue reading your synopsis or sample papers, then they probably won't leave your book due to the synopsis alone. But be careful, the synopsis is the only thing that tells your agent/publisher how the book will end, and please spoil the ending.

If your book is funny, enrich your manuscript with humour. Reading it should feel exciting, not like reading some sort of school report. But like I've said with a query letter, don't sweat too much over it. Yes, you should write several drafts, but don't just go bonkers on getting every single thing right.