“Best book” is a term used to describe a betting market which has the lowest “over-round” figure – or book percentage figure.
To understand what best book is all about, we first need to understand how a bookmaker makes a market and how that can vary from bookie to bookie, and what the term “over-round” is all about.
So, let’s consider a simplified example. In a five-runner horse race where each horse has an exactly equal chance of winning, the odds of each “should” be 4-1 in a completely fair world where the nice bookmaker isn’t looking to make a profit!
Let’s presume that this was the case with our five horse race and you placed £1 on each of the horses to win at 4-1. In this case, you would inevitably end up with £5 back from your initial outlay of £5, whichever horse won. In this hypothetical scenario, the bookmaker would be creating a perfectly “round book” as the five horses each have a 20% chance of winning, and their odds reflect this perfectly accurately (the five adding up to 100% of course).
Now let’s consider the not quite so nice bookie just down the road who prices each horse at 3-1 to win the very same race.
In this case, the implied probability of any particular horse winning is one in four or 25% according to the odds. And if we add the five percentages together we get to 125%. This bookmaker’s book is now described as being 25% "over-round".
So, the best book figure is the one which has the lowest over-round figure. And it is the over-round figure (sometimes called the Vigorish, "vig", or "juice") that gives the bookies their profit. This profit isn’t, of course, guaranteed, as the bookie may take a hit on one particular horse winning. The bookie also has to compete with other bookies and the exchanges for market share.
But, if the bookmaker’s average over round figure is 20%, they can expect to make around that figure as a long-term profit margin according to the laws of probability.
From a punter’s perspective, though, it makes sense to always try and locate the best book. This isn’t quick and easy to calculate, so fortunately, there is a dedicated page on Oddschecker devoted to identifying which markets have the "best book".