Hi the currect spec calls it discrete rather than decision. I think this is the only book for the course that is solely about discrete: www.amazon.co.uk/Level-Furthe...
AQA A Level Further Mathematics DiscreteBuy AQA A Level Further Mathematics Discrete by Geere, Nick (ISBN: 9781510433342) from Amazon's Book Store. Everyday low prices and free delivery on eligible orders.www.amazon.co.uk