I have both, but I rarely use the leaflet - tho I think it would help! Many swear by the leaflet.
About the book: it provides instructions based on your actual gauge (rather than if you are using a worsted or bulky yarn). It has these mini-charts which look daunting at first, but basically, you figure out what size project you want to knit and go across the chart and then you find your gauge and go down and there it tells you how many stitches to knit (or whatever). It’s a handy book.
For example, I want a sock with a foot circumfernce of 8.5" and my gauge is 6 sts per inch - it tells me to cast on 52 sts. Then it tells you how ling to knit the ribbing and the next chart tells you how many sts to knit to start the heel.