Perfectly agree with Roy and what he said.
Slap It is the bible of the slap technique but more than others book, it give you the way to improve and create your slap dictionary and your own voice.
Try to study any exercise from that book, plus analyze any possible variation in fingering and dynamic. When you study it try to have same dynamic and tone in each note you play.
It will give you the power to control the dynamic and, than you will be able to give your expression on what you playing, not the inversion.
Last thing: study at 60 of tempo, than when you're feel everything is good enough switch directly to 80 and eventually to 100. It will be the first step for a good time keeping.
If you can apply a good reading too, on what you're studying and don't limit yourself just on what you can hear from the audio cd, than you really reach all the goals of a deep and good studying.
Cheers. Enrico www.enricogaletta.com