diff --git a/README.md b/README.md
index a3fbf73..738108e 100644
--- a/README.md
+++ b/README.md
@@ -4,4 +4,4 @@ A Processing library wrapping JBox2D (http://www.jbox2d.org/).
 
 Download library: http://www.shiffman.net/p5/libraries/box2d_processing/box2d_processing.zip
 
-Tutorial and further examples for this library are available in The Nature of Code book: http://natureofcode.com.
+Tutorial and further examples for this library are available in The Nature of Code book: http://natureofcode.com. Note that there have been some changes to the library since the book was published; in particular you'll need to replace references to the `PBox2D` class with `Box2DProcessing`.