Include link to HtDP book and problem sets

README.md

@@ -178,6 +178,8 @@ All coursework under Core CS is **required**, unless otherwise indicated.
 `Ruby`
 `and more`
 
+The How to Code courses are based on the textbook [How to Design Programs](https://htdp.org/2003-09-26/). The First Edition is available for free online and includes problem sets and solutions. Students are encouraged to do these assignments.
+
 Courses | Duration | Effort | Prerequisites
 :-- | :--: | :--: | :--:
 [How to Code - Simple Data](https://www.edx.org/course/how-code-simple-data-ubcx-htc1x) | 7 weeks | 8-10 hours/week | none