Excellent books - you will not regret buying them. Definitely some of the best money I have spent on my training so far (Slight 'gambler effect' view there, but just think about the money spent on landing fees, medicals and all the rest).
To narrowly fail any subject will be an expensive and wearing experience. I look at any 'extra' study aids as a valuable addition to my library, and a wee extra peace of mind.
Best of luck
JH