Are you a group member? If so, the only book you need is your Food Optimising book which comes in the starter pack (it's an orange book).
Everything you need to know about the plans is in that book, and it will become your bible throughout your Slimming World Journey.
I've never bought any other books whilst on Slimming World - have never seen the need to. I have access to Slimming World online, Minimins, and my Consultants phone number if I ever need advice with Syns, recipes, etc. The only 'book' I ever buy is the monthly magazine, which is full of tips, motivational stories, recipes and much more.
You can successfully follow the plan without buying any additional books.