Remove 'early'. Kent taught the course through all of the 2000s I believe.

book.tex

@@ -124,7 +124,7 @@ those courses, Kent Dybvig, went on to build Chez
 Scheme~\citep{Dybvig:2006aa}, a production-quality and efficient
 compiler for Scheme. After completing his Ph.D. at the University of
 North Carolina, Kent returned to teach at Indiana University.
-Throughout the 1990's and early 2000's, Kent continued development of
+Throughout the 1990's and 2000's, Kent continued development of
 Chez Scheme and taught the compiler course.
 
 The compiler course evolved to incorporate novel pedagogical ideas