Marketed as Belle's first fictional novel, rather than a diary, Playing the Game simply reads as a continuation of her first two books. Packed full of witty quips and amusing anecdotes, fictional or not, this is a great follow-up book.
I don't think the book works as a stand-alone book though, as there are many, many references to past characters. You will get more amusement from this book if you know who Belle is referring to, and what has passed between the characters in question before. You'll still enjoy the book if you haven't read the others, but you just won't get the 'full' Belle experience.
However, if you have read and enjoyed The Intimate Adventures of a London Call Girl and The Further Adventures of a London Call Girl, you'll love this. It's told in the same simple yet refreshing writing style which means you can sail through the book without too much trouble and just enjoy it for what it is. I certainly did.