You can think of a synopsis as an Executive Summary of a book - it will tell you all of the key points, in a factual and non-biased or opinionated format. It is NOT a book review - we aren't telling you whether or not you should read the book, you can make your own mind up. We are just trying to help you make an informed decision based on a complete and accurate summary of the book.
Start by summarisng the key takeaways or lessons learnt, if it's non-fiction. For fictional content, it's a little trickier - you don't want necessarily want to give the game away, although a thorough book synopsis will often contain spoilers!