Re-word the "How to read this book" section (#1115)

index.Rmd

@@ -243,10 +243,12 @@ If you are new to both R and geographic data, do not be discouraged: we provide
 The book is divided into three parts:
 
 1. Part I: Foundations, aimed at getting you up-to-speed with geographic data in R.
-2. Part II: Extensions, which covers advanced techniques.
-3. Part III: Applications, to real-world problems.
+2. Part II: Advanced techniques, including spatial data visualization, bridges to GIS software, programming with spatial data, and statistical learning.
+3. Part III: Applications to real-world problems, including transportation, geomarketing and ecological modeling.
 
-The chapters get progressively harder in each so we recommend reading the book in order.
+The chapters get harder from one part to the next.
+We recommend reading all chapters in Part I in order before tackling the more advanced topics in Part II and Part III.
+The chapters in Part II and Part III benefit slightly from being read in order, but can be read independently if you are interested in a specific topic.
 A major barrier to geographical analysis in R is its steep learning curve.
 The chapters in Part I aim to address this by providing reproducible code on simple datasets that should ease the process of getting started.